STORMWATER C.3 GUIDEBOOK Stormwater Quality Requirements for Development Applications

Transcription

1 STORMWATER C.3 GUIDEBOOK Stormwater Quality Requirements for Development Applications Fourth Edition September 10, 2008 Visit for updates.

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3 Stormwater C.3 Guidebook Contra Costa Clean Water Program Don Freitas, Program Manager Tom Dalziel, Assistant Program Manager This Guidebook is referenced in stormwater ordinances adopted by Antioch Phil Hoffmeister Hercules Erwin Blancaflor Pinole Nancy Voisey Walnut Creek Rinta Perkins Brentwood Jeff Cowling Lafayette Donna Feehan Pittsburg Jason Burke Contra Costa County Rich Lierly Clayton Laura Hoffmeister Concord Jeff Roubal Martinez Alex Stroup Moraga Jill Mercurio Pleasant Hill Rod Wui Richmond Lynne Scarpa Contra Costa County Flood Control and Water Conservation District Greg Connaughton Danville Chris McCann Oakley Frank Kennedy San Pablo Karineh Samkian El Cerrito Garth Schultz Orinda Cathleen Terentieff San Ramon Steven Spedowfski Prepared with assistance from Dan Cloak Environmental Consulting Hydrograph Modification Management Plan consultants: Philip Williams & Associates and Brown & Caldwell 4 TH EDITION SEPTEMBER 10, 2008

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5 Table of Contents GLOSSARY HOW TO USE THIS GUIDEBOOK 1 Plan Ahead to Avoid the Three Most Common Mistakes 2 CHAPTER 1. POLICIES AND PROCEDURES 3 Thresholds and Start Dates 3 The 50% Rule for projects on Previously Developed Sites 4 Requirements for Projects not Subject to Provision C.3 4 Compliance Process at a Glance 5 Implementing C.3 on Phased Projects 5 Applying C.3 to New Subdivisions 7 Compliance with Flow-Control Requirements 8 Alternative Compliance Options 10 CHAPTER 2. CONCEPTS AND CRITERIA 11 Water-Quality Regulations 11 Maximum Extent Practicable 12 Best Management Practices 12 Hydrology for NPDES Compliance 13 Imperviousness 13 Sizing Requirements for Stormwater Treatment Facilities 14 Flow-Control (Hydrograph Modification Management) 15 Selection of Stormwater Treatment Facilities 16 Criteria for Infiltration Devices 17 Most LID Features and Facilities are Not Infiltration Devices 18 Environmental Benefit Perspective 18 CHAPTER 3. PREPARING YOUR STORMWATER CONTROL PLAN 21 Step by Step 23 Step 1: Assemble Needed Information 23 iii 4 th Edition September 2008

6 CONTRA COSTA CLEAN WATER PROGRAM Step 2: Identify Constraints & Opportunities 24 Step 3: Prepare and Document Your LID Design 24 Step 4. Specify Source Control BMPs 26 Identify Pollutant Sources 26 Note Locations on Stormwater Control Plan Exhibit 26 Prepare a Table and Narrative 26 Identify Operational Source Control BMPs 27 Step 5: Stormwater Facility Maintenance 27 Step 6: Stormwater Control Plan Report 28 Coordination with Site, Architectural, and Landscaping Plans 28 Construction Plan C.3 checklist 29 Certification 30 Sample Outline and Contents 30 Example Stormwater Control Plans 32 CHAPTER 4. LOW IMPACT DEVELOPMENT DESIGN GUIDE 33 Analyze Your Project for LID 34 Optimize the Site Layout 35 Use Pervious Surfaces 37 Disperse Runoff to Adjacent Pervious Areas 37 Direct runoff to Integrated Management Practices 38 Develop and Document Your Drainage Design 39 Step 1: Delineate Drainage Management Areas 40 Step 2: Classify DMAs and determine runoff factors 40 Step 3: Tabulate Drainage Management AReas 43 Step 4: Select and Lay Out IMPs on Site Plan 45 Step 6: Calculate Minimum Area and Volume of Each IMP 47 Step 7: Determine if IMP Area and Volume are adequate 48 Step 8: Computer Maximum Orifice Flow Rate 49 Step 9: Complete Your Summary Report 51 Specify Preliminary Design Details 53 Alternatives to LID Design 85 Treatment Control Alternatives 85 Treatment and Flow Control Alternatives 87 CHAPTER 5. OPERATION & MAINTENANCE OF STORMWATER FACILITIES 89 Stage 1: Ownership and Responsibility 91 Private Ownership and Maintenance 91 Transfer to Public Ownership 92 Stage 2: General Maintenance Requirements 93 Stage 3: Stormwater Facilities O&M Plan 93 4 th Edition September 2008 iv

7 STORMWATER C.3. GUIDEBOOK Tools and Assistance 94 Your O&M Plan: Step by Step 94 Step 1: Designate Responsible Individuals 94 Step 2: Summarize Drainage and BMPs 95 Step 3: Document Facilities As Built 96 Step 4: Prepare Customized Maintenance Plans 96 Step 5: Compile O&M Plan 97 Step 6: Updates 99 Stage 4: Interim Operation & Maintenance 99 Stage 5: Transfer Responsibility 100 Stage 6: Operation & Maintenance Verification 100 BIBLIOGRAPHY APPENDIX A: Local Exceptions and Additional Requirements Obtain from municipal planning staff. APPENDIX B: Soils, Plantings, and Irrigation for Bioretention Facilities APPENDIX C: Flow Control (Hydrograph Modification Management) Requirements APPENDIX D: Pollutant Sources/Source Control Checklist Figures FIGURE 4-1 SELF-TREATING AREAS FIGURE 4-2 SELF-RETAINING AREAS FIGURE 4-3 RELATIONSHIP OF IMPERVIOUS TO PERVIOUS AREA FIGURE 4-4 MORE THAN ONE DMA CAN DRAIN TO ONE IMP FIGURE 4-5 MORE THAN ONE DMA CAN DRAIN TO ONE IMP FIGURE 4-6 A, V1, and V Tables TABLE 1-1 THRESHOLDS, START DATES, AND REQUIREMENTS SUMMARIZED... 3 TABLE 1-2 OPTIONS FOR COMPLIANCE WITH FLOW-CONTROL REQUIREMENTS... 9 TABLE 3-1 FORMAT FOR TABLE OF SOURCE CONTROL MEASURES TABLE 3-2 CONSTRUCTION PLAN C.3 CHECKLIST TABLE 4-1 IDEAS FOR RUNOFF MANAGEMENT TABLE 4-2 RUNOFF FACTORS TABLE 4-3 FORMAT FOR TABULATING SELF-TREATING AREAS TABLE 4-4 FORMAT FOR TABULATING SELF-RETAINING AREAS v 4 th Edition September 2008

8 CONTRA COSTA CLEAN WATER PROGRAM TABLE 4-5 FORMAT FOR TABULATING AREAS DRAINING TO SELF-RETAINING AREAS. 44 TABLE 4-6 SIZING FACTORS TABLE 4-7 FORMAT FOR PRESENTING CALCULATIONS OF IMP AREAS AND VOLUMES 48 TABLE 5-1 SCHEDULE FOR PLANNING BMP OPERATION AND MAINTENANCE Checklists STORMWATER CONTROL PLAN CHECKLIST SOURCES AND SOURCE CONTROL BMP CHECKLIST... D-1 Design Sheets SELF-TREATING AND SELF-RETAINING AREAS PERVIOUS PAVEMENTS BIORETENTION FACILITIES FLOW-THROUGH PLANTERS DRY WELLS AND INFILTRATION FACILITIES th Edition September 2008 vi

9 STORMWATER C.3. GUIDEBOOK Stormwater Glossary Best Management Practice (BMP) Any procedure or device designed to minimize the quantity of pollutants that enter the storm drain system or to control stormwater flow. See Chapter Two. C.3 Provision, added in February 2003, of the San Francisco Bay Regional Water Quality Control Board s (see definition) stormwater NPDES permit (see definition). Requires each Discharger (see definition) to change its development review process to control the flow of stormwater and stormwater pollutants from new development sites. Order R C.3 Web Page California Association of Stormwater Quality Agencies (CASQA) California BMP Method Compensatory Mitigation Conditions of Approval (COAs) Contra Costa Clean Water Program (CCCWP) Design Storm Detention Publisher of the California Stormwater Best Management Practices Handbooks, available at Successor to the Storm Water Quality Task Force (SWQTF). A method for determining the required volume of stormwater treatment facilities. Described in Section of the California Stormwater Best Management Practice Manual (New Development) (CASQA, 2003). Treatment of an equivalent pollutant loading or quantity of stormwater runoff or other equivalent water quality benefit, created where no other requirement for treatment exists, in lieu of on-site treatment facilities. Requirements a municipality may adopt for a project in connection with a discretionary action (e.g., adoption of an EIR or negative declaration or issuance of a use permit). COAs may specify features required to be incorporated into the final plans for the project and may also specify uses, activities, and operational measures that must be observed over the life of the project. CCCWP is a collaboration established by an agreement among 19 Contra Costa cities and towns, Contra Costa County, and the Contra Costa County Flood and Water Conservation District. See list under Dischargers. CCCWP implements common tasks and assists the member agencies to implement their local stormwater pollution prevention programs. A hypothetical rainstorm defined by rainfall intensities and durations. The practice of holding stormwater runoff in ponds, vaults, within berms, or in depressed areas and letting it discharge slowly to the storm drain system. See definitions of infiltration and retention. vii 4 th Edition September 2008

10 CONTRA COSTA CLEAN WATER PROGRAM Directly Connected Impervious Area Direct Infiltration Dischargers Drawdown time Any impervious surface which drains into a catch basin, area drain, or other conveyance structure without first allowing flow across pervious areas (e.g. lawns). Infiltration via methods or devices, such as dry wells or infiltration trenches, designed to bypass unsaturated surface soils and transmit runoff directly to groundwater. The agencies named in the stormwater NPDES permit (see definition): Contra Costa County, Contra Costa County Flood Control and Water Conservation District, City of Clayton, City of Concord, Town of Danville, City of El Cerrito, City of Hercules, City of Lafayette, City of Martinez, Town of Moraga, City of Orinda, City of Pinole, City of Pittsburg, City of Pleasant Hill, City of Richmond, City of San Pablo, City of San Ramon, and City of Walnut Creek. In addition, three Contra Costa cities within the jurisdiction of the Central Valley Regional Water Quality Control Board have agreed to implement C.3 provisions under the same schedule: City of Antioch, City of Brentwood, and City of Oakley. The time required for a stormwater detention or infiltration facility to drain and return to the dry-weather condition. For detention facilities, drawdown time is a function of basin volume and outlet orifice size. For infiltration facilities, drawdown time is a function of basin volume and infiltration rate. Exemption Flow Control Group 1 Project Group 2 Project Head Hydrograph Exemption from the requirement to provide compensatory mitigation may be allowed for projects that meet certain criteria set by the RWQCB. These projects must, however, show impracticability (see definition of impracticable) of on-site treatment facilities and also show that the costs of compensatory mitigation would place an undue burden on the project. Control of runoff rates and durations as required by the CCCWP s Hydrograph Modification Management Plan. As described in NPDES Permit Provision C.3.c., a development project which creates or replaces an acre or more of impervious area. As described in NPDES Permit Provision C.3.c., a development project which creates or replaces between 10,000 square feet and one acre of impervious area. In hydraulics, energy represented as a difference in elevation. In slowflowing open systems, the difference in water surface elevation, e.g., between an inlet and outlet. Runoff flow rate plotted as a function of time. 4 th Edition September 2008 viii

11 STORMWATER C.3. GUIDEBOOK Hydrograph Modification Management Plan (HMP) Hydrologic Soil Group Impervious surface Impracticable Infeasible Indirect Infiltration Infiltration Infiltration Device Integrated Management Practice (IMP) Integrated Pest Management (IPM) Lead Agency Low Impact Development Maximum Extent Practicable (MEP) A Plan implemented by the dischargers so that post-project runoff from Group 1 Projects shall not exceed estimated pre-project rates and/or durations, where increased runoff would result in increased potential for erosion or other adverse impacts to beneficial uses. The HMP is available on the CCCWP s C.3 web page. Also see definition for flow control. Classification of soils by the Natural Resources Conservation Service (NRCS) into A, B, C, and D groups according to infiltration capacity. Any material that prevents or substantially reduces infiltration of water into the soil. See discussion of imperviousness in Chapter Two. As applied to on-site treatment facilities, technically infeasible (see definition) or excessively costly, as demonstrated by set criteria. As applied to on-site treatment facilities, impossible to implement because of technical constraints specific to the site. Infiltration via facilities, such as bioretention areas, expressly designed to treat runoff and then allow infiltration to surface soils. Seepage of runoff through soil to mix with groundwater. See definition of retention. Any structure that is designed to infiltrate stormwater into the subsurface and, as designed, bypasses the natural groundwater protection afforded by surface or near-surface soil. See definition for direct infiltration. A facility (BMP) that provides small-scale treatment, retention, and/or detention and is integrated into site layout, landscaping and drainage design. See Low Impact Development. An approach to pest management that relies on information about the life cycles of pests and their interaction with the environment. Pest control methods are applied with the most economical means and with the least possible hazard to people, property, and the environment. The public agency that has the principal responsibility for carrying out or approving a project. (California Environmental Quality Act Guidelines 15367). An integrated site design methodology that uses small-scale detention and retention (Integrated Management Practices, or IMPs) to protect water quality and mimic pre-existing site hydrological conditions. Standard, established by the 1987 amendments to the Clean Water Act, for the implementation of municipal stormwater pollution prevention programs (see definition). Also see Chapter Two. ix 4 th Edition September 2008

12 CONTRA COSTA CLEAN WATER PROGRAM National Pollutant Discharge Elimination System (NPDES) Numeric Criteria Operation and Maintenance (O&M) Permeable Pavements Percentile Rainfall Intensity Planned Unit Development (PUD) Rational Method Regional (or Watershed) Stormwater Treatment Facility Regional Water Quality Control Board (Regional Water Board or RWQCB) Retention Self-retaining area Self-treating area As part of the 1972 Clean Water Act, Congress established the NPDES permitting system to regulate the discharge of pollutants from municipal sanitary sewers and industries. The NPDES was expanded in 1987 to incorporate permits for stormwater discharges as well. Sizing requirements for stormwater treatment facilities established in Provision C.3.d. of the RWQCB s stormwater NPDES permit. Refers to requirements in the Stormwater NPDES Permit to inspect treatment BMPs and implement preventative and corrective maintenance in perpetuity. See Chapter Five. Pavements for roadways, sidewalks, or plazas that are designed to infiltrate runoff, including pervious concrete, pervious asphalt, unitpavers-on-sand, and crushed gravel. A method of determining design rainfall intensity. Storms occurring over a long period are ranked by rainfall intensity. The storm corresponding to a given percentile yields the design rainfall intensity. Allows land to be developed in a manner that does not conform to existing zoning requirements. Allows greater flexibility and innovation because the PUD is regulated as one unit rather than each component lot being regulated separately. A method of calculating runoff flows based on rainfall intensity, tributary area, and a factor representing the proportion of rainfall that runs off. A facility that treats runoff from more than one project or parcel. Participation in a regional facility may be in lieu of on-site treatment controls, subject to the requirements of NPDES permit provision C.3.g and the discretion of the local jurisdiction. California RWQCBs are responsible for implementing pollution control provisions of the Clean Water Act and California Water Code within their jurisdiction. There are nine California RWQCBs. Western and central Contra Costa County are under the jurisdiction of the RWQCB for the San Francisco Bay Region; eastern Contra Costa County is under the jurisdiction of the RWQCB for the Central Valley Region. The practice of holding stormwater in ponds or basins and allowing it to slowly infiltrate to groundwater. Some portion will evaporate. See definitions for infiltration and detention. An area designed to retain runoff. Self-retaining areas may include graded depressions with landscaping or pervious pavements. Natural, landscaped, or turf areas that drain overland off-site or to the storm drain system. 4 th Edition September 2008 x

13 STORMWATER C.3. GUIDEBOOK Source Control Stormwater Control Plan Stormwater Control Operation & Maintenance Plan Stormwater NPDES Permit A facility or procedure intended to prevent pollutants from entering runoff. A plan specifying and documenting permanent site features and facilities designed to control pollutants and stormwater flows for the life of the project. A plan detailing operation and maintenance requirements for stormwater treatment and flow-control facilities incorporated into a project. A permit issued by a Regional Water Quality Control Board (see definition) to local government agencies (Dischargers) placing provisions on allowable discharges of municipal stormwater to waters of the state. Storm Water Pollution Prevention Plan (SWPPP) Stormwater Pollution Prevention Program Treatment WEF Method A plan providing for temporary measures to control sediment and other pollutants during construction. A comprehensive program of activities designed to minimize the quantity of pollutants entering storm drains. See Chapter One. Removal of pollutants from runoff, typically by filtration or settling. A method for determining the minimum design volume of stormwater treatment facilities, recommended by the Water Environment Federation and American Society of Civil Engineers. Described in Urban Runoff Quality Management (WEF/ASCE, 1998). Water Board Water Quality Volume (WQV) See Regional Water Quality Control Board. For stormwater treatment facilities that depend on detention to work, the volume of water that must be detained to achieve maximum extent practicable pollutant removal. This volume of water must be detained for a specified drawdown time. xi 4 th Edition September 2008

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15 STORMWATER C.3. COMPLIANCE Start How to Use this Guidebook Read the Overview to get a general understanding of the requirements. Then follow the step-by-step instructions to prepare your Stormwater Control Plan. T HIS Guidebook will help you ensure that your project complies with the California Regional Water Quality Control Boards C.3 requirements. The requirements are complex and technical, and most applicants will require the assistance of a qualified civil engineer, architect, or landscape architect. Because every project is different, you should begin by scheduling a preapplication meeting with municipal planning staff. ICON KEY Helpful Tip Submittal Requirement Terms to Look Up To use the Guidebook, start by reviewing Chapter One to find out whether and how Provision C.3 applies to your project. Chapter One also provides an overview of the entire process of planning, design, construction, operation, and maintenance leading to compliance. References & Resources If there are terms and issues you find puzzling, try finding answers in the glossary or in Chapter Two. Chapter Two provides background on key stormwater concepts and water quality regulations, including design criteria. Then proceed to Chapter Three and follow the step-by-step guidance to prepare a Stormwater Control Plan for your site. Chapter Four, the Low Impact Development Design Guide, includes design procedures, calculation procedures, and instructions for presenting your design and calculations in your Stormwater Control Plan. In Chapter Five you ll find a detailed description of the process for ensuring operation and maintenance of your stormwater facilities over the life of the 1 4 th Edition September 2008

16 CONTRA COSTA CLEAN WATER PROGRAM project. The chapter includes step-by-step instructions for preparing a Stormwater Facilities Operation and Maintenance Plan. Local Requirements Cities, towns, or the County may have requirements that differ from, or are in addition to, this countywide Guidebook. See Appendix A for local requirements. Throughout each Chapter, you ll find references and resources to help you understand the regulations, complete your Stormwater Control Plan, and design stormwater control measures for your project. The most recent, updated version of the Guidebook, including updates and errata between editions, is on the Contra Costa Clean Water Program website. The on-line Guidebook is in Adobe Acrobat format. If you are reading the Acrobat version on a computer with an internet connection, you can use hyperlinks to navigate the document and to access various references. The hyperlinks are throughout the text, as well as in References and Resources sections (marked by the icon) and in the Bibliography. Some of these links (URLs) may be outdated. In that case, try entering portions of the title or other keywords into a web search engine. Construction-Phase Controls Your Stormwater Control Plan is a separate document from the Storm Water Pollution Prevention Plan (SWPPP). A SWPPP provides for temporary measures to control sediment and other pollutants during construction at sites that disturb one acre or more. See the CCCWP website for information on requirements for construction-phase controls. PLAN AHEAD TO AVOID THE THREE MOST COMMON MISTAKES The most common (and costly) errors made by applicants for development approvals with respect to C.3 compliance are: 1. Not planning for C.3 compliance early enough. You should think about your strategy for C.3 compliance before completing a conceptual site design or sketching a layout of subdivision lots (Chapter 3). 2. Assuming proprietary stormwater treatment facilities will be adequate for compliance. Most aren t (Chapter 2). 3. Not planning for periodic inspections and maintenance of treatment and flow-control facilities. Consider who will own and who will maintain the facilities in perpetuity and how they will obtain access, and identify which arrangements are acceptable to your municipality (Chapter 5). 4 th Edition September

17 STORMWATER C.3. COMPLIANCE Chapter 1 Policies and Procedures Determine if your development project must comply with the C.3 requirements, and review the steps to compliance. Thresholds and Start Dates Table 1-1 summarizes applicability of Provision C.3. TABLE 1-1. Thresholds, Start Dates, and Requirements summarized.* Threshold Start Date Requirement Group 1: Commercial, industrial, or residential developments that create one acre or more of impervious surface, and projects on previously developed sites that result in addition or replacement, which combined, total an acre or more of impervious surface. Group 2: Same as Group 1, but threshold reduced to 10,000 square feet impervious surface. Development applications deemed complete February 15, 2005 or later but before October 14, 2006 Development applications deemed complete October 14, 2006 or later Development applications deemed complete August 15, 2006 or later Treatment and source control measures as specified in the NPDES permit and this Guidebook. Treatment and source control measures, plus runoff flow control so post-project runoff does not exceed estimated pre-project rates or durations. Demonstrate compliance using one of the four options described in Appendix C. Treatment and source control measures as specified in the NPDES permit and this Guidebook. *Summary only. Requirements for any particular project are determined by your municipality. Projects consisting of one single family home not part of a larger common plan of development are excluded from the Group 2 definition. Requirements for such projects are as determined by local staff. 3 4 th Edition September 2008

18 CONTRA COSTA CLEAN WATER PROGRAM THE 50% RULE FOR PROJECTS ON PREVIOUSLY DEVELOPED SITES Projects on previously developed sites may also need to retrofit drainage to provide treatment of runoff from all impervious areas of the entire site. For sites creating or replacing a total amount of impervious area greater than the applicable Group 1 or Group 2 threshold (Table 1-1): If the new project results in an increase of, or replacement of, 50% or more of the previously existing impervious surface, and the existing development was not subject to stormwater treatment measures, then the entire project must be included in the treatment measure design. If less than 50% of the previously impervious surface is to be affected, only that portion must be included in the treatment measure design. Flow-control requirements apply to the site as a whole. The existing developed condition of a previously developed site is used as a baseline when determining if runoff rates or durations will increase as a result of the project. ICON KEY Helpful Tip Submittal Requirement Terms to Look Up References & Resources Interior remodels, routine maintenance or repair, roof or exterior surface replacement, repaving, road pavement structural section rehabilitation within the existing footprint, and any other reconstruction work within a public street or road right-of-way where both sides of that right-of-way are developed are not subject to C.3 requirements. REQUIREMENTS FOR PROJECTS NOT SUBJECT TO PROVISION C.3 These minimum standards for C.3 applicability are for the purpose of ensuring a consistent minimum level or floor for countywide implementation consistent with the requirements of the NPDES permit. Individual municipalities may choose a more expansive interpretation of the NPDES permit s applicability and may also choose to apply the C.3 source control, treatment, and flow-control requirements to projects that would be exempt under these minimum standards. Projects not subject to Provision C.3 may still be required to implement some or all of the requirements in this Guidebook. Requirements, which are determined by municipal staff, may include site design features such as minimizing impervious area and using permeable pavements and source controls. Treatment controls and flow controls may also be required for some or all impervious areas created or replaced by the project. 4 th Edition September

19 CHAPTER 1: OVERVIEW Compliance Process at a Glance For the applicant for development project approval, compliance follows these general steps: 1. Discuss C.3 requirements during a pre-application meeting with municipal staff. 2. Review the instructions in this Guidebook before you prepare your tentative map, preliminary site plan, drainage plan, and landscaping plan. 3. Prepare a Stormwater Control Plan and submit it with your application for development approvals (entitlements). 4. Following development approval, create your detailed project design, incorporating the features described in your Stormwater Control Plan. 5. In a table on your construction plans, list each stormwater control feature and facility and the plan sheet where it appears. 6. Prepare a draft Stormwater Facility Operation and Maintenance Plan and submit it with your application for building permits. Execute legal documents assigning responsibility for operation and maintenance of stormwater facilities. 7. Maintain stormwater facilities during construction and following construction in accordance with required warranties. 8. Following construction, submit a final Stormwater Facility Operation and Maintenance Plan and formally transfer responsibility for maintenance to the owner or permanent occupant. 9. The occupant or owner must periodically verify stormwater facilities are properly maintained. Implementing C.3 on Phased Projects Preparation of a complete and detailed Stormwater Control Plan is the key to cost-effective C.3 compliance and expeditious review of your project. Instructions for preparing a Stormwater Control Plan are in Chapter 3. When determining whether Provision C.3 requirements apply, a project should be defined consistent with CEQA definitions of project. That is, the project is the whole of an action which has the potential for adding or replacing or resulting 5 4 th Edition September 2008

20 CONTRA COSTA CLEAN WATER PROGRAM in the addition or replacement of roofs, pavement, or other impervious surfaces and thereby resulting in increased flows and stormwater pollutants. Whole of an action means the project may not be segmented or piecemealed into small parts if the effect is to reduce the quantity of impervious area for any part to below the C.3 threshold. Grandfathering. Municipalities may, at their discretion, exempt projects for which applications were deemed complete prior to the C.3 start dates (Table 1-1). However, this grandfathering applies only to the specific discretionary approval that was the subject of the pre-start-date application. Subsequent applications for further approvals constitute a project for the purposes of C.3. If those subsequent approvals or entitlements cover specific locations, modes, or designs for addition or replacement of roofs, pavement, or other impervious surfaces, and if the impervious area created or replaced is in excess of the applicable thresholds, then the C.3 requirements will apply to those areas of the project covered by the subsequent approval or entitlement. CEQA See the CCCWP s New Development web page for guidance on how to document stormwater impacts and mitigations in Initial Studies and Environmental Impact Reports. Consider for example an application for a subdivision tentative map which was deemed complete prior to the C.3 start dates. The project may be exempt from Provision C.3; however, if the project proponent later applies for discretionary approval of specific locations, modes, or designs of paving and structures, then C.3 requirements would apply to those improvements. Applying the 50% rule. Municipal staff will determine case-by-case when and how the 50% rule applies; in doing so staff may use the original entitlement (discretionary approval) as a guide when calculating the impervious area of the previously existing development. Stormwater Control Plan requirements for phased projects. Municipal staff may require, as part of an application for approval of a phased development project, a conceptual or master Stormwater Control Plan which describes and Local Requirements Cities, towns, or the County may have requirements that differ from, or are in addition to, this countywide Guidebook. See Appendix A and check with local planning and community development staff. illustrates, in broad outline, how the drainage for the project will comply with the Provision C.3 requirements. The level of detail in the conceptual or master Stormwater Control Plan should be consistent with the scope and level of detail of the development approval being considered. The conceptual or master Stormwater Control Plan should specify that a more detailed Stormwater Control Plan for each later phase or portion of the project will be submitted with subsequent applications for discretionary approvals. 4 th Edition September

21 CHAPTER 1: OVERVIEW Applying C.3 to New Subdivisions If a tentative map approval would potentially entitle future owners to construct new or replaced impervious area which, in aggregate, could exceed of the thresholds (Table 1-1), then the applicant must take steps to ensure C.3 requirements can and will be implemented as the subdivision is built out. If the tentative map application does not include plans for site improvements, the applicant should nevertheless identify the type, size, location, and final ownership of stormwater treatment and flow-control facilities adequate to serve new roadways and any common areas, and to also manage runoff from an expected reasonable estimate of the square footage of future roofs, driveways, and other impervious surfaces on each individual lot. The municipality may condition approval of the map on implementation of stormwater treatment measures in compliance with Provision C.3 when construction occurs on the individual lots. This condition may be enforced by a grant deed of development rights or by a development agreement. If a municipality deems it necessary, the future impervious area of one or more lots may be limited by a deed restriction. This might be necessary when a project is exempted from one or all C.3 provisions because the total impervious area is below a threshold, or to ensure runoff from impervious areas added after the project is approved does not overload a stormwater treatment and flow-control facility. Subdivision maps should dedicate an open space easement, as defined by Government Code Section 51075, to suitably restrict the future building of structures at each stormwater facility location. In general, it is recommended stormwater treatment facilities not be located on individual single-family residential lots, particularly when those facilities manage runoff from other lots, from streets, or from common areas. However, local requirements vary. A better alternative may be to locate stormwater facilities on one or more separate, jointly owned parcels. See the Policy for C.3 Compliance for Subdivisions on the Contra Costa Clean Water Program s C.3 web page. After consulting with local planning staff, applicants for subdivision approvals will propose one of the following four options, depending on project characteristics and local policies: 1. Show the sum of future impervious areas to be created on all parcels could not exceed the C.3 thresholds shown in Table th Edition September 2008

22 CONTRA COSTA CLEAN WATER PROGRAM 2. Show that, for each and every lot, the intended use can be achieved with a design which disperses runoff from roofs, driveways, streets, and other impervious areas to self-retaining pervious areas, using the criteria in Chapter 4 of this Guidebook. 3. Prepare improvement plans showing drainage to treatment and/or flow-control facilities designed in accordance with this Guidebook, and commit to constructing the facilities prior to transferring the lots. 4. Prepare improvement plans showing drainage to treatment and/or flow-control facilities designed in accordance with this Guidebook, and provide appropriate legal instruments to ensure the proposed facilities will be constructed and maintained by subsequent owners. For the option selected, municipal staff will determine the appropriate conditions of approval, easements, deed restrictions, or other legal instruments necessary to assure future compliance. Compliance with Flow-Control Requirements As shown in Table 1-1, in addition to incorporating treatment controls, projects creating or replacing an acre or more of impervious area must also provide flow control so post-project runoff does not exceed estimated pre-project rates and durations. Projects subject to flow-control requirements have four options for demonstrating compliance. The options are summarized in Table 1-2. Detailed requirements are in Appendix C. Depending on location and existing site conditions, a project proponent may wish to consider the feasibility of these options in the following order: For projects on previously developed sites, it may be possible to show the project will not increase the existing quantity of impervious area and will not facilitate the efficiency of drainage collection and conveyance (Option 1). Depending on project location, the project proponent may be able show all downstream channels between the project site and the Bay/Delta are enclosed pipes, are engineered hardened channels, are subject to tidal action, or are aggrading (Option 4a). Proponents may use the LID Design Guide in Chapter 4 to meet both treatment and flow-control requirements (Option 2). Proponents of larger developments, particularly those with complex or extensive drainage, might consider creating a continuous hydrologic 4 th Edition September

23 CHAPTER 1: OVERVIEW simulation model, using the criteria in Appendix C, to demonstrate that, after incorporation of flow-control measures, post-project runoff will not exceed pre-project rates or durations (Option 3). Under Options 4b and 4c, proponents may propose and implement an appropriate in-stream restoration project to fully mitigate the potential risk of increased downstream erosion created by their proposed development. TABLE 1-2. Options for compliance with flow-control requirements* What must be demonstrated Option 1: No increase in impervious area Option 2: Integrated Management Practices Option 3: Post-project runoff does not exceed preproject rates and durations Option 4a: All downstream reaches are at low risk of erosion Options 4b and 4c: Erosion risks are mitigated by in-stream restoration projects How applicants can comply Compare the project design to the pre-project condition and show the project will not increase impervious area and also will not increase efficiency of drainage collection and conveyance. Use the design procedure and design criteria in this Guidebook, and the Program s sizing tool, to select and size IMPs for flow control (also meets treatment requirements). Use a continuous-simulation model and 30 years or more of hourly rainfall data to simulate pre-project and post-project runoff, including the effect of proposed control facilities. Show all downstream channels between the project site and the Bay/Delta are enclosed pipes, are engineered hardened channels, are subject to tidal action, or are aggrading. Propose and implement appropriate in-stream restoration projects to fully mitigate potential risk. Stormwater Control Plan submittal requirements Inventory and accounting of existing and proposed impervious areas, measures used to reduce imperviousness, and a qualitiative comparison of preand post-project drainage efficiency. Stormwater Control Plan and sizing tool output (Chapter 3). Model parameters and modeling techniques are specified in Appendix C. Report or letter report by an engineer or qualified environmental professional documenting drainage between the project site and the Bay or Delta. Requires additional regulatory approvals. See Appendix C. *Summary only. Applicability to and requirements for any particular project are determined by your municipality. 9 4 th Edition September 2008

24 CONTRA COSTA CLEAN WATER PROGRAM Runoff treatment is required regardless of the flow-control compliance option chosen. Alternative Compliance Options Provision C.3.g of the stormwater NPDES permit contains the Regional Water Board s requirements for alternative compliance. Under certain conditions, a project applicant may reduce or omit treatment facilities from their project and create an equivalent water-quality benefit at a different site. Where feasible, this site should be in the same drainage basin. Other C.3 requirements including site designs to minimize imperviousness and source control BMPs will still apply. The options for alternative compliance are: 1. Participate in a regional stormwater treatment facility. 2. Demonstrate the impracticability of incorporating treatment BMPs on your development site and also demonstrate how you will provide compensatory mitigation (equivalent treatment or equivalent waterquality benefit) at another site. 3. Obtain an exemption from the requirements if impracticability of incorporating treatment BMPs on your site can be established, the costs of participating in a regional facility or implementing compensatory mitigation at another site would unduly burden the project, and the project is a redevelopment project that also meets certain categorical criteria established by the Water Board. Experience has shown implementation of LID facilities, as described in Chapter 4, is feasible on nearly all development sites with sufficient advance planning. Also see the discussion of Selection of Stormwater Treatment Facilities in Chapter 2. Determinations of eligibility for alternative compliance will be made by local development review staff. References and Resources: Appendix C Flow Control RWQCB Order No. R (Stormwater NPDES Permit Amendments) RWQCB Order No. R , revising hydrograph modification management (flow-control) requirements CCCWP Policy for C.3 Compliance for Subdivisions CCCWP Web Page for Construction Activities CCCWP Hydrograph Modification Management Plan 4 th Edition September

25 STORMWATER C.3. COMPLIANCE Chapter 2 Concepts and Criteria Technical background and explanations of policies and design requirements T he Regional Water Board first issued a municipal stormwater NPDES permit to Contra Costa County, its 19 cities and towns, and the Contra Costa Flood Control and Water Conservation District in The permit mandates a comprehensive program to prevent stormwater pollution. That program now includes street sweeping, maintenance of storm drains, identification and elimination of illicit discharges to storm drains, business inspections, public outreach, construction site inspections, monitoring and studies of stream health, and control of runoff pollutants from new developments and redevelopments. The Regional Water Board added Provision C.3 in 2003, and the permittees began implementing the provision in The Regional Water Board added hydrograph modification management (flow control) requirements in The CCCWP has created a Low Impact Development (LID) approach that ensures consistent and thorough implementation of the Regional Water Board s requirements. This chapter explains the technical background of the CCCWP s LID approach and how it was derived. Water-Quality Regulations Provision C.3 requires municipalities to condition development approvals with incorporation of specified stormwater controls. The municipalities annual report to the Regional Water Board includes a list of development projects approved during the year and the specific stormwater controls required for each project. In the annual report, the municipalities also document their program to verify stormwater treatment and flow-control facilities are being adequately maintained. The municipalities not the Regional Board or its staff are charged with 11 4 th Edition September 2008

26 CONTRA COSTA CLEAN WATER PROGRAM ensuring development projects comply with the C.3 requirements. (Regional Water Board staff sometimes reviews stormwater controls in connection with applications for Clean Water Act Section 401 water-quality certification, which is required for projects that involve work in streams, including dredging and filling.) In a nutshell, Provision C.3 requires that applicable new developments and redevelopments: Design the site to minimize imperviousness, detain runoff, and infiltrate runoff where feasible Cover or control sources of stormwater pollutants Treat runoff prior to discharge from the site Ensure runoff does not exceed pre-project peaks and durations Maintain treatment and flow-control facilities MAXIMUM EXTENT PRACTICABLE Clean Water Act Section 402(p)(3)(iii) sets the standard for stormwater controls as maximum extent practicable, but doesn t define that term. As implemented, maximum extent practicable is ever-changing and varies with conditions. Many stormwater controls, including LID, have proven to be practicable in most development projects. To achieve fair and effective implementation, criteria and guidance for those controls must be detailed and specific while also offering the right amount of flexibility or exceptions for special cases. The NPDES permit includes various standards, including hydrologic criteria, which have been found to comprise maximum extent practicable. CCCWP s C.3 guidance is continuously improved and refined based on the experience of municipal planners and engineers, with input from land developers and development professionals. BEST MANAGEMENT PRACTICES Clean Water Act Section 402(p) and USEPA regulations (40 CFR ) specify a municipal program of management practices to control stormwater pollutants. Best Management Practice (BMP) refers to any kind of procedure or device designed to minimize the quantity of pollutants that enter the storm drain system. To minimize confusion, this guidebook refers to facilities, features, or controls to be incorporated into development projects. All of these are BMPs. 4 th Edition September

27 CHAPTER 2: STORMWATER CONCEPTS Hydrology for NPDES Compliance IMPERVIOUSNESS Schueler (1995) proposed imperviousness as a unifying theme for the efforts of planners, engineers, landscape architects, scientists, and local officials concerned with urban watershed protection. Schueler argued (1) that imperviousness is a useful indicator linking urban land development to the degradation of aquatic ecosystems, and (2) imperviousness can be quantified, managed, and controlled during land development. Imperviousness has long been understood as the key variable in urban hydrology. Peak runoff flow and total runoff volume from small urban catchments is usually calculated as a function of the ratio of impervious area to total area (rational method). The ratio correlates to the composite runoff factor, usually designated C. Increased flows resulting from urban development tend to increase the frequency of small-scale flooding downstream. Imperviousness links urban land development to degradation of aquatic ecosystems in two ways. First, the combination of paved surfaces and piped runoff efficiently collects urban pollutants and transports them, in suspended or dissolved form, to surface waters. These pollutants may originate as airborne dust, be washed from the atmosphere during rains, or may be generated by automobiles and outdoor work activities. Second, increased peak flows and runoff durations typically cause erosion of stream banks and beds, transport of fine sediments, and disruption of aquatic habitat. Measures taken to control stream erosion, such as hardening banks with riprap or concrete, may permanently eliminate habitat. By reducing infiltration to groundwater, imperviousness may also reduce dry-weather stream flows. Imperviousness has two major components: rooftops and transportation (including streets, highways, and parking areas). The transportation component is usually larger and is more likely to be directly connected to the storm drain system. The effects of imperviousness can be mitigated by disconnecting impervious areas from the drainage system and by making drainage less efficient that is, by encouraging detention and retention of runoff near the point where it is generated. Detention and retention reduce peak flows and volumes and allow pollutants to settle out or adhere to soils before they can be transported downstream th Edition September 2008

28 CONTRA COSTA CLEAN WATER PROGRAM SIZING REQUIREMENTS FOR STORMWATER TREATMENT FACILITIES NPDES permit criteria for sizing stormwater treatment facilities and flow-control facilities are based on simulation of runoff from a long-term (30-year or more) rainfall record. This is different from the event-based or design storm hydrology typically used to size drainage and flood-control facilities. The CCCWP s LID design guidance (Chapter 4) was crafted to ensure LID facilities comply with the NPDES permit s hydraulic sizing requirements for stormwater treatment facilities and flow-control facilities. The technical background follows. Most runoff is produced by frequent storms of small or moderate intensity and duration. Treatment facilities are designed to treat smaller storms and the first flush of larger storms approximately 80% of average annual runoff. The NPDES permit identifies two types of treatment facilities volume-based and flow-based. For volume-based treatment facilities, the permit references two alternative methods: the WEF method and the California BMP method. As described in Chapter 4, local rainfall data and the California BMP method are used for sizing detention basins in Contra Costa County. Both the WEF and California BMP methods are based on continuous simulation of runoff ICON KEY Helpful Tip Submittal Requirement Terms to Look Up References & Resources from a hypothetical one-acre area entering a basin designed to draw down in 48 hours. The simulation is iterated to find the unit basin size that detains about 80% of the total runoff during the simulation period. The unit basin storage size is expressed as a rainfall depth. The depth varies from about 0.45" to 0.85" in Contra Costa County. For flow-based facilities, the NPDES permit specifies the rational method be used to determine flow. The rational method uses the equation Q = CiA, where Q = flow C = weighted runoff factor between 0 and 1 i = rainfall intensity A = area The permit identifies three alternatives for calculating rainfall intensity: 4 th Edition September

29 CHAPTER 2: STORMWATER CONCEPTS 1. the intensity-duration-frequency method, with a hydrograph corresponding to a 50-year storm, 2. the 85 th percentile rainfall intensity times two, and inches per hour. An analysis conducted for the CCCWP determined all three methods yielded similar results. The 0.2 inches per hour rainfall intensity is used for sizing flowbased treatment facilities in Contra Costa County. This intensity corresponds to a storm depth of approximately 0.6 inches. The CCCWP used the 0.2 inches per hour criterion to develop a consistent countywide sizing factor for bioretention facilities when used for stormwater treatment only (i.e., not for flow control). The factor is based on maintaining a minimum percolation rate of 5 inches per hour through an engineered soil mix. The sizing factor is the ratio of the design intensity of rainfall on tributary impervious surfaces (0.2 inches/hour) to the design percolation rate in the facility (5 inches/hour), or 0.04 (dimensionless). FLOW-CONTROL (HYDROGRAPH MODIFICATION MANAGEMENT) The NPDES permit specifies for applicable projects: estimated post-project runoff peaks and durations do not exceed estimated pre-project peaks and durations if increased stormwater runoff peaks or durations could cause erosion or other significant effects on beneficial uses. Applicants may select among four options for compliance. See Table 1-2. The first three options demonstrate runoff will not exceed pre-project durations by showing there will be no net increase in impervious area, by using Integrated Management Practice designs and sizing factors developed by the Program, or by constructing a site-specific hydrologic model. The fourth option is to demonstrate that, even though runoff will increase, it will not cause erosion or other significant effects on beneficial uses. This may be done by showing downstream channels are not susceptible to erosion (Option 4a) or that a restoration project will mitigate any impacts from increased flows (Options 4b and 4c). Details on compliance requirements are in Appendix C. Technical background is in the Hydrograph Modification Management Plan, which is available on the Program s website th Edition September 2008

30 CONTRA COSTA CLEAN WATER PROGRAM Selection of Stormwater Treatment Facilities As required by the NPDES permit, Contra Costa municipalities require project proponents design and implement stormwater treatment measures to reduce the discharge of stormwater pollutants to the maximum extent practicable. The CCCWP has determined * the following types of facilities can meet the maximum extent practicable standard: Bioretention facilities, flow-through planters, and other facilities using filtration through soil or sand (sized with a surface area at least 0.04 times the effective impervious tributary area). Dry wells, infiltration trenches, infiltration basins, and other facilities using infiltration to native soils (in locations where soils and tributary land uses are appropriate). Extended detention basins, constructed wetlands or other facilities using settling (sized according to the volume-based criterion, with a detention time of 48 hours). Lack of space, in itself, is not a suitable justification for using a less-effective treatment, because the uses of the site and the site design can be altered as needed to accommodate bioretention facilities. In most cases, these effective facilities can be fit into required landscaping setbacks, easements, or other unbuildable areas. Where possible, drainage to inlets, and drainage away from overflows and underdrains, should be by gravity. Where site topography makes it infeasible to accommodate gravity-fed facilities in the project design, the design flow may be captured in a vault or sump and pumped via force main to an effective facility. The following situations sometimes present special challenges: Portions of sites which are not being developed or redeveloped, but which must be retrofit to meet treatment requirements in accordance with the 50% rule. Sites smaller than one acre approved for lot-line to lot-line development or redevelopment as part of a municipality s stated objective to preserve or enhance a pedestrian-oriented smart-growth type of urban design. * Contra Costa Clean Water Program, Policy on Selection of Stormwater Treatment Facilities for Maximum Extent Practicable Effectiveness in Compliance with NPDES Provision C.3, March 21, th Edition September

31 CHAPTER 2: STORMWATER CONCEPTS In these special situations, the following types of facilities should each be evaluated in priority order (depending on the specific characteristics of the site and as determined by the municipal stormwater coordinator) until a feasible design is found: 1. Bioretention facilities or planter boxes fed by gravity. 2. Capture of the design flow in a vault or sump and pumping to bioretention facilities or planter boxes. 3. A subsurface sand or media filter with a maximum design surface loading rate of 5 inches per hour and a minimum media depth of 18 inches. The sand surface must be made accessible for periodic inspection and maintenance (for example, via a removable grate). 4. A higher-rate surface biofilter, such as a tree-pit-style unit. The grading and drainage design should minimize the area draining to each unit and maximize the number of discrete drainage areas and units. 5. A higher-rate vault-based filtration unit. Regional Water Board staff has found oil/water separators ( water quality inlets ) and storm drain inlet filters do not meet the maximum extent practicable standard. * When used as a sole method of stormwater treatment, hydrodynamic separators, including vortex separators and continuous deflection separators ( CDS units ), do not meet the maximum extent practicable requirement, although they may be used in series with other facilities. Criteria for Infiltration Devices The NPDES permit restricts the design and location of infiltration devices that, as designed, may bypass filtration through surface soils before reaching groundwater. These devices include: Infiltration basins. * Use of Storm Drain Inlet Filters and Oil/Water Separators to Meet the Requirements of NPDES Municipal Stormwater Permits, letter from Regional Water Board Executive Officer Bruce Wolfe to Bay Area Stormwater Management Agencies Association managers, August 5, 2004 Policy on the Use of Hydrodynamic Separators to Achieve Compliance with NPDES Provision C.3, November 16, th Edition September 2008

32 CONTRA COSTA CLEAN WATER PROGRAM Infiltration trenches (includes french drains). Unlined retention basins (i.e., basins with no outlets). Unlined or open-bottomed vaults or boxes installed below grade (dry wells). Infiltration devices may not be used in: areas of industrial or light industrial activity; areas subject to high vehicular traffic (25,000 or greater average daily traffic on main roadway or 15,000 or more average daily traffic on any intersecting roadway); automotive repair shops; car washes; fleet storage areas (bus, truck, etc.); nurseries; other areas with pollutant sources that could pose a threat to groundwater, as determined by municipal staff. The vertical distance from the base of any infiltration device to the seasonal high groundwater mark shall be at least 10 feet. Infiltration devices shall be located a minimum of 100 feet horizontally from any known water supply wells. In addition, infiltration devices are not recommended where: The infiltration device would receive drainage from areas where chemicals are used or stored, where vehicles or equipment are washed, or where refuse or wastes are handled. Surface soils or groundwater are polluted. The facility could receive sediment-laden runoff from disturbed areas or unstable slopes. Increased soil moisture could affect the stability of slopes of foundations. Soils are insufficiently permeable to allow the device to drain within 72 hours. 4 th Edition September

33 CHAPTER 2: STORMWATER CONCEPTS MOST LID FEATURES AND FACILITIES ARE NOT INFILTRATION DEVICES Self-treating and self-retaining areas, pervious pavements, bioretention facilities, and flow-through planters are not considered to be infiltration devices because they do not bypass filtration through surface soils before reaching groundwater. Bioretention facilities work by percolating runoff through 18 inches or more of engineered soil. This removes most pollutants before the runoff is allowed to seep into native soils below or discharge through the outlet. Further pollutant removal typically occurs in the unsaturated (vadose) zone before moisture reaches groundwater. Self-treating and self-retaining areas allow removal of pollutants in surface soils before runoff mixes with groundwater. Where there is concern about the effects of increased soil moisture on slopes or foundations, an impermeable barrier may be added so the facility is flow through and all treated runoff is underdrained away from the facility. See the design sheets for Bioretention Facilities and Flow-Through Planters in Chapter 4. Environmental Benefit Perspective The diverse natural geography of Contra Costa County includes tidal and freshwater wetlands, alluvial plains, and mountain slopes. Average annual rainfall varies from 12.5 inches in Brentwood to 30 inches in Orinda. The climate, soils, slope, and vegetation give each Contra Costa stream a characteristic structure of riffles, pools, terraces, floodplains, and wetlands. In relatively undisturbed stream reaches, this geomorphic structure supports trees and other riparian vegetation. Trees provide shade (cooling stream temperatures), create root wads and undercut banks (refuge for fish) and produce falling leaves and detritus (the bottom of a food web). Fish, frogs, and other animals have evolved to thrive in riparian habitats. Because Contra Costa habitats are diverse and complex, some species are specialized, have limited ranges, and may be rare. Contra Costa s landscape, like that of all the San Francisco Bay Area, has been repeatedly transformed since the Spanish arrived in the 1770s. Even before the area was developed, European grasses, weeds, and other plants replaced much of the native vegetation. Creek flows were diverted to irrigate farms, and wetlands were diked or filled for farmland. Suburbs and former farm towns developed rapidly during and after the Second World War. In many places, to make flood-prone land suitable for development, creeks were channelized or confined within levees. Buildings, streets, and pavement now cover much of the land, and storm drains pipe runoff from urban neighborhoods directly into the creeks. Urbanization has changed the timing and intensity of stream flows and has set off a chain of unanticipated consequences th Edition September 2008

34 CONTRA COSTA CLEAN WATER PROGRAM These consequences include more frequent flooding, destabilized stream banks, armoring of streambanks with riprap and concrete, loss of streamside trees and vegetation, and the destruction of stream habitat. The remaining habitat, even where it has been disturbed and reduced to remnants, is an important refuge for various species. The U.S. and California have listed some of these species, including steelhead (Oncorhyncus mykiss), as endangered. Other species are listed as threatened, rare, or having other special status. Once altered, natural streams and their ecosystems cannot be fully restored. However, it is possible to stop, and partially reverse, the trend of declining habitat and preserve some ecosystem values for the benefit of future generations. This is an enormous, long-term effort. Managing runoff from a single development site may seem inconsequential, but by changing the way most sites are developed (and redeveloped), we may be able to preserve and enhance existing stream ecosystems in urban and urbanizing areas. References and Resources The Importance of Imperviousness (Tom Scheuler, 1995) Site Planning for Urban Stream Protection, available from the Center for Watershed Protection) California Stormwater BMP Handbooks Urban Runoff Quality Management, Water Environment Federation and American Society of Civil Engineers, ISBN ISBN Policy on the Use of Hydrodynamic Separators to Achieve Compliance with NPDES Provision C.3, November 16, 2005 Policy on Selection of Stormwater Treatment Facilities for Maximum Extent Practicable Effectiveness in Compliance with NPDES Provision C.3 (CCCWP, 2007) Use of Storm Drain Inlet Filters and Oil/Water Separators to Meet the Requirements of NPDES Municipal Stormwater Permits, letter from Regional Water Board Executive Officer Bruce Wolfe to Bay Area Stormwater Management Agencies Association managers, August 5, 2004 Stormwater Infiltration, Bruce K. Ferguson, ISBN San Francisco Bay RWQCB Order No (Stormwater NPDES Permit) CCCWP Stormwater Management Plan ( ) Central Valley RWQCB Order (Stormwater NPDES Permit covering Antioch, Brentwood, and Oakley and eastern portions of unincorporated Contra Costa County) San Francisco Bay RWQCB Order No. R (Stormwater NPDES Permit C.3 Amendment) San Francisco Bay RWQCB Order No. R (revising flow-control (hydrograph modification management) requirements RWQCB Water Quality Control Plan for the San Francisco Bay Basin (Basin Plan) RWQCB Water Quality Control Plan for the Central Valley Region (Basin Plan) Clean Water Act Section 402(p) 40 CFR (d)(2)(iv)(A)(2) Stormwater Regulations for New Development Restoring Streams in Cities (Riley, 1998) Stream Restoration: Principles, Processes, and Practices (Federal Interagency Stream Restoration Working Group, 1998, revised 2001) Contra Costa County Watershed Atlas (Contra Costa County, th Edition September

35 STORMWATER C.3. COMPLIANCE Chapter 3 Preparing Your Stormwater Control Plan Step-by-step assistance to document compliance. Y our Stormwater Control Plan will demonstrate your project complies with all applicable requirements in the stormwater NPDES permit to minimize imperviousness, retain or detain stormwater, slow runoff rates, incorporate required source controls, treat stormwater prior to discharge from the site, control runoff rates and durations if required, and provide for operation and maintenance of treatment and flowcontrol facilities. ICON KEY * Helpful Tip Submittal Requirement Terms to Look Up References & Resources The Plan must be submitted with your application for discretionary approvals and must have sufficient detail to ensure the stormwater design, site plan, and landscaping plan are congruent. A complete and thorough Stormwater Control Plan will facilitate quicker review and fewer cycles of review. Every Contra Costa municipality requires a Stormwater Control Plan for every applicable project. Your Stormwater Control Plan will consist of a report and an exhibit. Municipal staff will use the following checklist to evaluate your Plan: * Additional requirements may apply if runoff from your site discharges directly to creeks, wetlands, or the Bay/Delta. (See Order R , Provision C.3.b.ii) th Edition September 2008

36 CONTRA COSTA CLEAN WATER PROGRAM STORMWATER CONTROL PLAN CHECKLIST CONTENTS OF EXHIBIT Show all of the following on drawings: Existing natural hydrologic features (depressions, watercourses, relatively undisturbed areas) and significant natural resources. (Step 1 in the following step-by-step instructions) Soil types and depth to groundwater. (Step 1) Existing and proposed site drainage network and connections to drainage off-site. (Step 3) Proposed design features and surface treatments used to minimize imperviousness. (Step 3) Entire site divided into separate drainage areas, with each area identified as self-treating, self-retaining (zero-discharge), draining to a self-retaining area, or draining to an IMP. (Step 3) For each drainage area, types of impervious area proposed (roof, plaza/sidewalk, and streets/parking) and area of each. (Step 3) Proposed locations and sizes of treatment or flow-control facilities. (Step 3) Potential pollutant source areas, including refuse areas, outdoor work and storage areas, etc. listed in Appendix D and corresponding required source controls. (Step 4) CONTENTS OF REPORT Include all of the following in a report: Narrative analysis or description of site features and conditions that constrain, or provide opportunities for, stormwater control. (Step 2) Narrative description of site design characteristics that protect natural resources. (Step 3) Narrative description and/or tabulation of site design characteristics, building features, and pavement selections that minimize imperviousness of the site. (Step 3) Tabulation of proposed pervious and impervious area, showing self-treating areas, self-retaining areas, and areas tributary to each treatment or flow-control facility. (Step 3) Preliminary designs, including calculations, for each treatment, or flow-control facility. Elevations should show sufficient hydraulic head for each. (Step 3) A table of identified pollutant sources and for each source, the source control measure(s) used to reduce pollutants to the maximum extent practicable. See worksheet in Appendix D. (Step 4) General maintenance requirements for infiltration, treatment, and flow-control facilities. (Step 5) Means by which facility maintenance will be financed and implemented in perpetuity. (Step 5) Statement accepting responsibility for interim operation & maintenance of facilities. (Step 5) Identification of any conflicts with codes or requirements or other anticipated obstacles to implementing the Stormwater Control Plan. (Step 6) Construction Plan C.3 Checklist. (Step 6) Certification by a civil engineer, architect, and landscape architect. (Step 6) Appendix: Compliance with flow-control requirements (if using an HMP compliance option other than Option 2, Integrated Management Practices). 4 th Edition September

37 CHAPTER 3: STORMWATER CONTROL PLAN Step by Step Suggested coordination with site and landscape design Begin with general project requirements and program. Sketch conceptual site layout, building locations, and circulation. Revise site layout, building locations, and circulation to accommodate LID design. Develop landscaping plan. Submit Site Plan, Landscape Plan, and Stormwater Control Plan Plan and design your stormwater controls integrally with the site planning and landscaping for your project. It s best to start with general project requirements and preliminary site design concepts; then prepare the detailed site design, landscape design, and Stormwater Control Plan simultaneously. This will help ensure that your site plan, landscape plan, and Stormwater Control Plan are congruent. The following step-by-step procedure should optimize your design by identifying the best opportunities for stormwater controls early in the design process. The recommended steps are: 1. Assemble needed information. 2. Identify site opportunities and constraints. 3. Follow the LID design guidance in Chapter 4 to analyze your project for LID and to develop and document your drainage design. 4. Specify source controls using the table in Appendix D. 5. Plan for ongoing maintenance of treatment and flow-control facilities. 6. Complete the Stormwater Control Plan. Municipal staff may recommend you prepare and submit a preliminary site design prior to formally applying for planning and zoning approvals. Your preliminary site design should incorporate a conceptual plan for site drainage, including selftreating and self-retaining areas and the location and approximate sizes of any treatment and flow-control facilities. This additional up-front design effort will save time and avoid potential delays later in the review process. Step 1: Assemble Needed Information To select types and locations of treatment and flow-control facilities, the designer needs to know the following site characteristics: Existing natural hydrologic features and natural resources, including any contiguous natural areas, wetlands, watercourses, seeps, or springs. Existing site topography, including contours of any slopes of 4% or steeper, general direction of surface drainage, local high or low points or depressions, any outcrops or other significant geologic features th Edition September 2008

38 CONTRA COSTA CLEAN WATER PROGRAM Zoning, including requirements for setbacks and open space. Soil types (including hydrologic soil groups) and depth to groundwater, which may determine whether infiltration is a feasible option for managing site runoff. Depending on site location and characteristics, and on the selection of treatment and flow-control facilities, site-specific information (e.g. from boring logs or geotechnical studies) may be required. Existing site drainage. For undeveloped sites, this should be obtained by inspecting the site and examining topographic maps and survey data. For previously developed sites, site drainage and connection to the municipal storm drain system can be located from site inspection, municipal storm drain maps, and plans for previous development. Existing vegetative cover and impervious areas, if any. References and Resources The Importance of Imperviousness (Scheuler 1995). Start at the Source (BASMAA 1999), p. 36 Step 2: Identify Constraints & Opportunities Review the information collected in Step 1. Identify the principal constraints on site design and selection of treatment and flow-control facilities as well as opportunities to reduce imperviousness and incorporate facilities into the site and landscape design. For example, constraints might include impermeable soils, high groundwater, groundwater pollution or contaminated soils, steep slopes, geotechnical instability, high-intensity land use, heavy pedestrian or vehicular traffic, utility locations, or safety concerns. Opportunities might include existing natural areas, low areas, oddly configured or otherwise unbuildable parcels, easements and landscape amenities including open space and buffers (which can double as locations for bioretention facilities), and differences in elevation (which can provide hydraulic head). Prepare a brief narrative describing site opportunities and constraints. This narrative will help you as you proceed with LID design and explain your design decisions to others. Step 3: Prepare and Document Your LID Design Use the Low Impact Development Design Guide (Chapter 4) to analyze your project for LID, design and document drainage, and specify preliminary design details for integrated management practices. 4 th Edition September

39 CHAPTER 3: STORMWATER CONTROL PLAN Chapter 4 includes calculation procedures and formats for presenting your calculations. As shown in the checklist (page 22), your Exhibit must show: The entire site divided into separate drainage areas, with each area identified as self-treating, self-retaining, draining to a self-retaining area, or draining to an IMP. Each area should be clearly marked with a unique identifier. For each drainage area, the types of impervious area proposed, and the area of each. Proposed locations and sizes of treatment and flow-control facilities. Each facility should be clearly marked with a unique identifier. Your Stormwater Control Plan report must include: Tabulation of proposed self-treating areas, self-retaining areas, areas draining to self-retaining areas, and areas draining to IMPs, and the corresponding IMPs identified on the Exhibit. Calculations, in the format shown in Chapter 4, showing the minimum square footage required and proposed square footage for each IMP. Preliminary designs for each IMP. The design sheets and accompanying drawings in Chapter 4 may be used or adapted for this purpose. Also include in your Stormwater Control Plan report: A narrative overview of your design and how your design decisions optimize the site layout, use pervious surfaces, disperse runoff from impervious surfaces, and drain impervious surfaces to engineered IMPs. See Chapter 4. A narrative briefly describing each DMA, its drainage, and where drainage will be directed. A narrative briefly describing each IMP. Include any special characteristics or features distinct from the design sheets in Chapter 4. Group and consolidate descriptions, or provide additional detail, as necessary to help the reviewer understand your drainage design th Edition September 2008

40 CONTRA COSTA CLEAN WATER PROGRAM References and Resources Chapter 4 Start at the Source (BASMAA, 1999). Your municipality s General Plan Your municipality s Zoning Ordinance and Development Codes Low Impact Development Manual (Prince George s County, Maryland, 1999). Bioretention Manual (Prince George s County, Maryland, rev. 2002) Low Impact Development Technical Guidance Manual for Puget Sound (Puget Sound Action Team, 2005) LID for Big Box Retailers (Low Impact Development Center, 2006) Step 4. Specify Source Control BMPs Some everyday activities such as trash recycling/disposal and washing vehicles and equipment generate pollutants that tend to find their way into storm drains. These pollutants can be minimized by applying source control BMPs. Source control BMPs include permanent, structural features that may be required in your project plans such as roofs over and berms around trash and recycling areas and operational BMPs, such as regular sweeping and housekeeping, that must be implemented by the site s occupant or user. The maximum extent practicable standard typically requires both types of BMPs. In general, operational BMPs cannot be substituted for a feasible and effective permanent BMP. Use the following procedure to specify source control BMPs for your site: IDENTIFY POLLUTANT SOURCES Review the first column in the Pollutant Sources/Source Control Checklist (Appendix D). Check off the potential sources of pollutants that apply to your site. NOTE LOCATIONS ON STORMWATER CONTROL PLAN EXHIBIT Note the corresponding requirements listed in Column 2 of the Pollutant Sources/Source Control Checklist (Appendix D). Show the location of each pollutant source and each permanent source control BMP in your Stormwater Control Plan Exhibit. PREPARE A TABLE AND NARRATIVE Check off the corresponding requirements listed in Column 3 in the Pollutant Sources/Source Control Checklist (Appendix D). Now, create a table using the format in Table 3-1. In the left column, list each potential source on your site (from Appendix E, Column 1). In the middle column, list the corresponding permanent, structural BMPs (from Columns 2 and 3, Appendix D) used to prevent pollutants from entering runoff. Accompany this table with a narrative that explains any special features, materials, or methods of construction that will be used to implement these permanent, structural BMPs. 4 th Edition September

41 CHAPTER 3: STORMWATER CONTROL PLAN TABLE 3-1. Format for table of permanent and operational source control measures. Potential source of runoff pollutants Permanent source control BMPs Operational source control BMPs IDENTIFY OPERATIONAL SOURCE CONTROL BMPS To complete your table, refer once again to the Pollutant Sources/Source Control Checklist (Appendix D, Column 4). List in the right column of your table the operational BMPs that should be implemented as long as the anticipated activities continue at the site. The local stormwater ordinance requires that these BMPs be implemented; the same BMPs may also be required as a condition of a use permit or other revocable discretionary approval for use of the site. References and Resources Appendix D, Stormwater Pollutant Sources/Source Control Checklist RWQCB Order R , Provision C.3.k Start at the Source, Section 6.7: Details, Outdoor Work Areas California Stormwater Industrial/Commercial Best Management Practice Handbook Urban Runoff Quality Management (WEF/ASCE, 1998) Chapter 4: Source Controls Step 5: Stormwater Facility Maintenance As required by NPDES Permit Provision C.3.e, your local municipality will periodically verify that treatment and flow-control facilities on your site are maintained and continue to operate as designed. To make this possible, your municipality will require that you include in your Stormwater Control Plan: 1. A means to finance and implement facility maintenance in perpetuity. 2. Acceptance of responsibility for maintenance from the time the facilities are constructed until responsibility for operation and maintenance is legally transferred. A warranty covering a period following construction may also be required. 3. An outline of general maintenance requirements for the treatment and flow-control facilities you have selected th Edition September 2008

42 CONTRA COSTA CLEAN WATER PROGRAM Your local municipality will also require that you prepare and submit a detailed Stormwater Facilities Operation and Maintenance Plan that sets forth a maintenance schedule for each of the treatment and flow-control facilities built on your site. An agreement assigning responsibility for maintenance and providing for inspections and certification o Details of these requirements, and instructions for preparing a Stormwater Facilities Operation and Maintenance Plan, are in Chapter 5. References and Resources Chapter 5 Model Stormwater Ordinance (CCCWP, 2005) Operation, Maintenance, and Management of Stormwater Management Systems (Watershed Management Institute, 1997) Step 6: Stormwater Control Plan Report Your Stormwater Control Plan Report should document the information gathered and decisions made in Steps 1-5. A clear, complete, well-organized report will make it possible to confirm your design meets the minimum requirements of the NPDES permit, the municipal stormwater pollution prevention ordinance, and this Guidebook. COORDINATION WITH SITE, ARCHITECTURAL, AND LANDSCAPING PLANS Before completing your Stormwater Control Plan exhibit and report, ensure your stormwater control design is fully coordinated with the site plan, grading plan, and landscaping plan being proposed for the site. Information submitted and presentations to design review committees, planning commissions, and other decision-making bodies must incorporate relevant aspects of the stormwater design. In particular, ensure: Curb elevations, elevations, grade breaks, and other features of the drainage design are consistent with the delineation of DMAs. The top edge (overflow) of each bioretention facility is level all around its perimeter this is particularly important in parking lot medians. The resulting grading and drainage design is consistent with the design for parking and circulation. Bioretention facilities and other IMPs do not create conflicts with pedestrian access between parking and building entrances. 4 th Edition September

43 CHAPTER 3: STORMWATER CONTROL PLAN Vaults and utility boxes will be accommodated outside bioretention facilities and will not be placed within bioretention facilities. The visual impact of stormwater facilities, including planter boxes at building foundations and any terracing or retaining walls required for the stormwater control design, is shown in renderings and other architectural drawings. Landscaping plans, including planting plans, show locations of bioretention facilities, and the plant requirements are consistent with the engineered soils and conditions in the bioretention facilities. Renderings and representation of street views incorporate any stormwater facilities located in street-side buffers and setbacks. Any potential conflicts with local development standards have been identified and resolved. TABLE CONSTRUCTION PLAN C.3 CHECKLIST When you submit construction plans for City review and approval, the plan checker will compare that submittal with your Stormwater Control Plan. By creating a Construction Plan C.3 Checklist for your project, you will facilitate the plan checker s comparison and speed review of your project Format for Construction Plan C.3 Checklist. Stormwater Control Plan Page # BMP Description See Plan Sheet #s Here s how: 1. Create a table similar to Table 3-2. Number and list each measure or BMP you have specified in your Stormwater Control Plan in Columns 1 and 2 of the table. Leave Column 3 blank. Incorporate the table into your Stormwater Control Plan th Edition September 2008

44 CONTRA COSTA CLEAN WATER PROGRAM 2. When you submit construction plans, duplicate the table (by photocopy or electronically). Now fill in Column 3, identifying the plan sheets where the BMPs are shown. List all plan sheets on which the BMP appears. Submit the updated table with your construction plans. Note that the updated table or Construction Plan C.3 Checklist is only a reference tool to facilitate comparison of the construction plans to your Stormwater Control Plan. Local municipal staff can advise you regarding the process required to propose changes to the approved Stormwater Control Plan. CERTIFICATION Your local municipality may require that your Stormwater Control Plan be certified by an architect, landscape architect, or civil engineer. See Appendix A. Your certification should state: The selection, sizing, and preliminary design of stormwater treatment and other control measures in this plan meet the requirements of Regional Water Quality Control Board Order R and subsequent amendments. SAMPLE OUTLINE AND CONTENTS I. Project Setting A. Project Name, Location, Description B. Existing site features and conditions C. Opportunities and constraints for stormwater control II. Low Impact Development Design Strategies A. Optimization of site layout (1) Limitation of development envelope (2) Preservation of natural drainage features (3) Setbacks from creeks, wetlands, and riparian habitats (4) Minimization of imperviousness (5) Using drainage as a design element B. Use of permeable pavements C. Dispersal of runoff to pervious areas 4 th Edition September

45 CHAPTER 3: STORMWATER CONTROL PLAN D. Use of Integrated Management Practices III. Documentation of Drainage Design A. Drainage Management Areas (1) Tabulation (2) Descriptions B. Integrated Management Practices (1) Tabulation and Sizing Calculations (2) Descriptions IV. Source Control Measures A. Description of site activities and potential sources of pollutants B. Table showing sources, permanent source controls, and operational source controls V. Facility Maintenance Requirements A. Ownership and responsibility for maintenance in perpetuity. (1) Commitment to execute any necessary agreements and/or annex into a fee mechanism, per local requirements. (2) Statement accepting responsibility for operation and maintenance of facilities until that responsibility is formally transferred. B. Summary of maintenance requirements for each stormwater facility. VI. VII. Construction Plan C.3 Checklist Certifications Attachment: Stormwater Control Plan Exhibit Appendix: Compliance with Flow-Control (Hydrograph Modification Management) requirements (if IMPs are not used) th Edition September 2008

46 CONTRA COSTA CLEAN WATER PROGRAM EXAMPLE STORMWATER CONTROL PLANS Example Stormwater Control Plans can be accessed via the CCCWP s New Development/C.3 web pages. Your Stormwater Control Plan will reflect the unique character of your own project and should meet the requirements identified in this Guidebook. Municipal staff can assist you to determine how specific requirements apply to your project. 4 th Edition September

47 STORMWATER C.3. COMPLIANCE Chapter 4 Low Impact Development Design Guide Guidance for designing and documenting your LID site drainage, stormwater treatment facilities, and flow-control facilities Y our Stormwater Control Plan to be submitted with your application for planning and zoning approvals (entitlements) must show how your project will comply with applicable stormwater treatment and flowcontrol (hydrograph modification management) standards. This will require careful documentation of: Pervious and impervious areas in the planned project. Drainage from each of these areas. Locations, sizes, and types of proposed treatment and flow-control facilities. This Low Impact Development Design Guide will help you: Your Stormwater Control Plan must include calculations showing the site drainage and proposed treatment and flow-control facilities meet the criteria in this Guidebook. Analyze your project and identify and select options for implementing LID techniques to meet runoff treatment requirements and flowcontrol requirements, if they apply th Edition September 2008

48 CONTRA COSTA CLEAN WATER PROGRAM Design and document drainage for the whole site and document how that design meets this Guidebook s stormwater treatment and flowcontrol criteria. Specify preliminary design details and integrate your LID drainage design with your paving and landscaping design. Alternatives to LID design are discussed in the final section of this chapter. Before beginning your LID design, determine ICON KEY whether flow-control requirements apply to your site. Helpful Tip See Table 1.1 in Chapter 1. If flow-control requirements apply, review Appendix C to Submittal Requirement understand your options for meeting those Terms to Look Up requirements. The calculation procedures in this References & Resources Design Guide enable you to comply with flowcontrol requirements using Option 2 in Appendix C. If flow-control requirements do not apply, or if you are using another option to meet flow-control requirements, then you may use the treatment-only factors to size your facilities. Analyze Your Project for LID Conceptually, there are four LID strategies for managing runoff from buildings and paving: 1. Optimize the site layout by preserving natural drainage features and designing buildings and circulation to minimize the amount of roofs and paving. 2. Use pervious surfaces such as turf, gravel, or pervious pavement or use surfaces that retain rainfall, such as green roofs. 3. Disperse runoff from impervious surfaces on to adjacent pervious surfaces (e.g., direct a roof downspout to disperse runoff onto a lawn). 4. Drain impervious surfaces to engineered Integrated Management Practices (IMPs), such as bioretention facilities, flow-through planters, or dry wells. IMPs infiltrate runoff to groundwater and/or percolate runoff through engineered soil and allow it to drain away slowly. 4 th Edition September

49 CHAPTER 4 : LID DESIGN GUIDE A combination of two or more strategies may work best for your project. Table 4-1 includes ideas for applying LID strategies to site conditions and types of development. With forethought in design, the four LID strategies can provide multiple, complementary benefits to your development. Pervious surfaces reduce heat island effects and temperature extremes. Landscaping improves air quality, creates a better place to live or work, and upgrades value for rental or sale. Retaining natural hydrology helps preserve and enhance the natural character of the area. LID drainage design can also conserve water and reduce the need for drainage infrastructure. OPTIMIZE THE SITE LAYOUT To minimize stormwater-related impacts, apply the following design principles to the layout of newly developed and redeveloped sites: Define the development envelope and protected areas, identifying areas that are most suitable for development and areas that should be left undisturbed. Set back development from creeks, wetlands, and riparian habitats. Preserve significant trees. Where possible, conform the site layout along natural landforms, avoid excessive grading and disturbance of vegetation and soils, and replicate the site s natural drainage patterns. Concentrate development on portions of the site with less permeable soils, and preserve areas that can promote infiltration. Coordination Chapter One includes a presentation of how review of your project s site design and landscape design is coordinated with review for compliance with Provision C.3. For all types of development, limit overall coverage of paving and roofs. This can be accomplished by designing compact, taller structures, narrower and shorter streets and sidewalks, smaller parking lots (fewer stalls, smaller stalls, and more efficient lanes), and indoor or underground parking. Examine site layout and circulation patterns and identify areas where landscaping can be substituted for pavement th Edition September 2008

50 CONTRA COSTA CLEAN WATER PROGRAM TABLE 4-1. Ideas for Runoff Management Site Features and Design Objectives Green Roof Self-retaining Areas Pervious Pavement Bioretention Facility Flow-through Planter Dry Well Cistern with bioretention Clayey native soils Permeable native soils Very steep slopes Shallow groundwater Avoid saturating subsurface soils Connect to roof downspouts Parking lot islands and medians Sites with extensive landscaping Densely developed sites with limited space/landscape Flat sites with poor drainage Fit IMPs into landscape and setback areas Make drainage a design feature Convey as well as treat stormwater 4 th Edition September

51 CHAPTER 4 : LID DESIGN GUIDE Detain and retain runoff throughout the site. On flatter sites, it typically works best to intersperse landscaped areas and IMPs among the buildings and paving. On hillside sites, drainage from upper areas may be collected in conventional catch basins and piped to landscaped areas and IMPs in lower areas. Or use low retaining walls to create terraces that can accommodate IMPs. Use drainage as a design element. Use depressed landscape areas, vegetated buffers, and bioretention areas as amenities and focal points within the site and landscape design. Bioretention areas can be almost any shape and should be located at low points. When configured as swales, bioretention areas can detain and treat low runoff flows and also convey higher flows. USE PERVIOUS SURFACES Consider a green roof. Although not yet widely used in California, green roofs are growing in popularity. Potential benefits include longer roof life, lower heating and cooling costs, and better sound insulation, in addition to air quality and water quality benefits. For C.3 compliance purposes, green roofs are considered not to produce increased runoff or runoff pollutants (i.e., any runoff from a green roof requires no further treatment or detention). For more information on green roofs, see Consider permeable pavements and surface treatments. Inventory paved areas on your preliminary site plan. Identify where permeable pavements, such as crushed aggregate, turf block, unit pavers, pervious concrete, or pervious asphalt could be substituted for impervious concrete or asphalt paving. DISPERSE RUNOFF TO ADJACENT PERVIOUS AREAS Look for opportunities to direct runoff from impervious areas to adjacent landscaping. The design, including slopes and soils, must reflect a reasonable expectation that an inch of rainfall will soak into the soil and produce no runoff. For example, a lawn or garden depressed 3-4" below surrounding walkways or driveways provides a simple but functional landscape design element. For sites subject to stormwater treatment requirements only, a 2:1 maximum ratio of impervious to pervious area is acceptable. If flow-control requirements apply, the impervious-to-pervious ratio must be limited to 1:1. Be sure soils will drain adequately. Under some circumstances, it may be allowable to direct runoff from impervious areas to pervious pavement (for example, from roof downspouts to a parking lot paved with crushed aggregate or turf block). The pore volume of pavement and base course must be enough to retain an inch of rainfall, including runoff from the tributary area. The slopes and soils must be compatible with infiltrating that volume without producing runoff th Edition September 2008

52 CONTRA COSTA CLEAN WATER PROGRAM DIRECT RUNOFF TO INTEGRATED MANAGEMENT PRACTICES The CCCWP has developed design criteria for the following IMPs: Bioretention facilities, which can be configured as swales, free-form areas, or planters to integrate with your landscape design. Flow-through planters, which can be used near building foundations and other locations where infiltration to native soils is not desired. Dry wells and other infiltration facilities, which can be used only where soils are permeable. See restrictions on page 17. Cisterns, in combination with a bioretention facility. See the design sheets at the end of this chapter. Finding the right location for treatment and flow-control facilities on your site involves a careful and creative integration of several factors: To make the most efficient use of the site and to maximize aesthetic value, integrate IMPs with site landscaping. Many local zoning codes may require landscape setbacks or buffers, or may specify that a minimum portion of the site be landscaped. It may be possible to locate some or all of your site s treatment and flow-control facilities within this same area, or within utility easements or other non-buildable areas. Planter boxes and bioretention facilities must be level or nearly level all the way around. Linear bioretention facilities (swales) may be gently sloped end to end, but opposite sides must be at the same elevation. For effective, low-maintenance operation, locate facilities so drainage into and out of the device is by gravity flow. Pumped systems are feasible, but are expensive, require more maintenance, are prone to untimely failure, and can cause mosquito control problems. Most IMPs require 3 feet or more of head. If the property is being subdivided now or in the future, the facility should be in a common, accessible area. In particular, avoid locating facilities on private residential lots. Even if the facility will serve only one site owner or operator, make sure the facility is located for ready access by inspectors from the local municipality and the Contra Costa Mosquito and Vector Control District. The facility must be accessible to equipment needed for its maintenance. Access requirements for maintenance will vary with 4 th Edition September

53 CHAPTER 4 : LID DESIGN GUIDE the type of facility selected. Bioretention facilities will typically need access for the same types of equipment used for landscape maintenance. To complete your analysis, include in your Stormwater Control Plan a brief narrative documenting the site layout and site design decisions you made. This will provide background and context for how your design meets the quantitative LID design criteria. Develop and Document Your Drainage Design The CCCWP s design documentation procedure begins with careful delineation of pervious areas and impervious areas (including roofs) throughout the site. The procedure accounts for how runoff from each delineated area is managed. For areas draining to IMPs, the procedure ensures each IMP is appropriately sized. The procedure results in a space-efficient, cost-efficient LID design for meeting C.3 requirements on most residential and commercial/industrial developments. The procedure arranges documentation of drainage design and IMP sizing in a consistent format for presentation and review. This procedure is intended to facilitate, not substitute for, creative interplay among site design, landscape design, and drainage design. Several iterations may be needed to optimize your drainage design as well as aesthetics, circulation, and use of available area for your site. You should be able to complete the needed calculations using only the project s site development plan, hydrologic soil group (A, B, C, or D) and mean annual precipitation. Mean annual precipitation at locations in Contra Costa County can be determined using isohyetal maps accessible from the Program s C.3 web page. The Program has created an IMP Sizing Calculator to facilitate the iterative calculations needed to create an optimal site design. The calculator is a stand-alone application and is available, along with instructions for its use, on the CCCWP s C.3 web pages. In addition to performing calculations, the IMP Sizing Calculator formats calculation results into a summary report. The summary report can be attached to your Stormwater Control Plan submittal. The following formulas and procedures can be used without the sizing calculator to complete calculations and prepare a report suitable for submittal with your Stormwater Control Plan. The same formulas and procedures should be used to check and verify calculations made with the IMP Sizing Calculator th Edition September 2008

54 CONTRA COSTA CLEAN WATER PROGRAM STEP 1: DELINEATE DRAINAGE MANAGEMENT AREAS This is the key first step. You must divide the entire project area into individual, discrete Drainage Management Areas (DMAs). Typically, lines delineating DMAs follow grade breaks and roof ridge lines. The Exhibit, tables, text, and calculations in your Stormwater Control Plan will illustrate, describe, and account for runoff from each of these areas. Use separate DMAs for each surface type (e.g., landscaping, pervious paving, or roofs). Each DMA must be assigned a single hydrologic soil group. Assign each DMA an identification number and determine its size in square feet. FIGURE 4-1. Self-treating areas are entirely pervious and drain directly off-site or to the storm drain system. STEP 2: CLASSIFY DMAS AND DETERMINE RUNOFF FACTORS Next, determine how drainage from each DMA will be handled. Each DMA will be one of the following four types: 1. Self-treating areas. 2. Self-retaining areas (also called zerodischarge areas). 3. Areas that drain to self-retaining areas. 4. Areas that drain to IMPs. Self-treating areas are landscaped or turf areas that do not drain to IMPs, but rather drain directly off site or to the storm drain system. Examples include upslope undeveloped areas which are ditched and drained around a development and grassed slopes that drain off-site to an existing public street or storm drain. In general, self-treating areas include no impervious areas, unless the impervious area is very small (5% or less) in relationship to the receiving pervious area and slopes are gentle enough to ensure runoff from impervious areas will be absorbed into the vegetation and soil. Self-retaining areas are designed to retain the first one inch of rainfall without producing any runoff. The technique works best on flat, heavily landscaped sites. It may be used on mild slopes if there is a reasonable expectation that a one-inch rainfall event would produce FIGURE 4-2. Self-retaining areas. Berm or depress the grade to retain at least an inch of rainfall and set inlets of any area drains at least 3 inches above low point to allow ponding. Rationale Pollutants in rainfall and windblown dust will tend to become entrained in the vegetation and soils of landscaped areas, so no additional treatment is needed. It is assumed the self-treating landscaped areas will produce runoff less than or equal to the preproject site condition. 4 th Edition September

55 CHAPTER 4 : LID DESIGN GUIDE no runoff. To create self-retaining turf and landscape areas in flat areas or on terraced slopes, berm the area or depress the grade into a concave cross-section so that these areas will retain the first inch of rainfall. Grade slopes, if any, toward the center of the pervious area. Inlets of area drains, if any, should be set 3 inches above the low point to allow ponding. Under some circumstances, pervious pavement (e.g., crushed stone, pervious asphalt, or pervious concrete) can be selfretaining. Adjacent roofs or impervious FIGURE 4-3. Relationship of impervious to pervious area for self-retaining areas. Where flow-control requirements apply: pervious impervious Where only treatment requirements apply : pervious ½ impervious pavement may drain on to the pervious pavement in the same maximum ratios as described below. A gravel base course four or more inches deep will ensure an adequate proportion of rainfall is infiltrated into native soils Derivation of Criteria A computer model was used to continuously simulate rainfall, infiltration, and runoff at an hourly time-step over 30 years. Results indicate drainage areas using the 1:1 ratio will not exceed preproject peaks and durations. (including clay soils) rather than producing runoff. Consult with a qualified engineer regarding infiltration rates, pavement stability, and suitability for the intended traffic. Drainage from green roofs is considered to be self-retained. An emergency overflow should be provided for extreme events. Areas draining to self-retaining areas. Runoff from impervious or partially pervious areas can be managed by routing it to self-retaining pervious areas. For example, roof downspouts can be directed to lawns, and driveways can be sloped toward landscaped areas. The maximum ratio is 2 parts impervious area for every 1 part pervious area if only treatment requirements apply to the development project. If flow-control requirements also apply, the maximum ratio is 1 part impervious area for every 1 part pervious area. The drainage from the impervious area must be directed to and dispersed within the pervious area, and the entire area must be designed to retain an inch of rainfall without flowing off-site. For example, if the maximum ratio of 2 parts impervious area into 1 part pervious area is used, then the pervious area must absorb 3 inches of water over its surface before overflowing to an off-site drain. A partially pervious area may be drained to a self-retaining area. For example, a driveway composed of unit pavers may drain to an adjacent lawn. In this case, the maximum ratios are: 41 4 th Edition September 2008

56 CONTRA COSTA CLEAN WATER PROGRAM (Runoff factor) x (tributary area) 2 x (self-retaining area) Equation 4-1 For treatment-only sites, and (Runoff factor) x (tributary area) 1 x (self-retaining area) Equation 4-2 for sites subject to flow-control requirements. Use the runoff factors in Table 4-2. Prolonged ponding is a potential problem at higher impervious/pervious ratios. In your design, ensure that the pervious area soils can handle the additional run-on and are sufficiently well-drained. Runoff from self-treating and self-retaining areas does not require any further treatment or flow control. TABLE 4-2. Runoff factors to be used when sizing IMPs. Surface Treatment and Flow Control Treatment only Roofs Concrete or Asphalt Pervious Concrete Porous Asphalt Grouted Unit Pavers Solid Unit Pavers Crushed Aggregate Turfblock Landscape, Group A Soil Landscape, Group B Soil Landscape, Group C Soil Landscape, Group D Soil Areas draining to IMPs are used to calculate the required size of the IMP. On most densely developed sites such as commercial and mixed-use developments and small-lot residential subdivisions most DMAs will drain to IMPs. The CCCWP has developed sizing factors (ratios of IMP area to impervious DMA area). For each IMP design, factors are provided for: 4 th Edition September

57 CHAPTER 4 : LID DESIGN GUIDE Treatment-only. Treatment-plus-flow-control. Treatment-only IMPs are smaller and in some cases are simpler in design. FIGURE 4-4. MORE THAN ONE Drainage Management Area can drain to a single IMP. More than one drainage management area can drain to the same IMP. However, because the minimum IMP sizes are determined by ratio to drainage area size, a drainage area may not drain to more than one IMP. See Figures 4-4 and 4-5. Where possible, design site drainage so only impervious roofs and pavement drain to IMPs. This yields a simpler, more efficient design and also helps protect IMPs from becoming clogged by sediment. If it is necessary to include turf, landscaping, or pervious pavements within the area draining to an IMP, list each surface as a separate DMA. A runoff factor (similar to a C factor used in the rational method) is applied to account for the reduction in the quantity of runoff. For example, when a turf or landscaped drainage management area drains to an IMP, the resulting increment in IMP size is: FIGURE 4-5. ONE DRAINAGE Management Area cannot rain to more than one IMP. Use a grade break to divide the DMA. (pervious area) (runoff factor) (sizing factor). Use the runoff factors in Table 4-2. STEP 3: TABULATE DRAINAGE MANAGEMENT AREAS Tabulate self-treating areas in the format shown in Table 4-3. Tabulate self-retaining areas in the format shown in Table 4-4. Tabulate areas draining to self-retaining areas in the format shown in Table 4-5. Check to be sure the total amount of (square feet of tributary area runoff factor) for all DMAs draining to a receiving selfretaining area is no greater than a 1:1 ratio to the square footage of the receiving self-retaining area itself. A 2:1 ratio may be used on sites not subject to flow-control requirements th Edition September 2008

58 CONTRA COSTA CLEAN WATER PROGRAM Compile a list of DMAs draining to IMPs. Proceed to Step 4 to check the sizing of the IMPs. TABLE 4-3. Format for Tabulating Self-Treating Areas DMA Name Area (square feet) TABLE 4-4. Format for Tabulating Self-Retaining Areas DMA Name Area (square feet) TABLE 4-5. Format for Tabulating Areas Draining to Self-Retaining Areas DMA Name Area (square feet) Postproject surface type Runoff factor Product (Area x runoff factor)[a] Receiving selfretaining DMA Receiving selfretaining DMA Area (square feet) [B] Ratio [A]/[B] 4 th Edition September

59 CHAPTER 4 : LID DESIGN GUIDE STEP 4: SELECT AND LAY OUT IMPS ON SITE PLAN Select from the list of IMPs in Table 4-6. Descriptions, illustrations, designs, and design criteria for the IMPs are in the design sheets at the end of this chapter. Note the configuration of area (A), surface reservoir volume (V1) and subsurface reservoir volume (V2) for bioretention facilities and flow-through planters as shown in Figure 4-6. Once you have laid out the IMPs, calculate the square footage you have set aside on your site plan for each IMP. FIGURE 4-6. A, V 1, and V th Edition September 2008

60 CONTRA COSTA CLEAN WATER PROGRAM TABLE 4-6. Sizing Factors Treatment and Flow Control NRCS Soil Group IMP A B C D Bioretention Facility A V V 2 N/A N/A Flow-through Planter A N/A N/A V 1 N/A N/A V 2 N/A N/A Dry Well A N/A N/A V N/A N/A Cistern + bioretention facility A (bioretention facility) V (cistern) * Cistern sized for flow control when used in conjunction with a treatment IMP. IMP underdrain required in B, C and D soils. Treatment Only Bioretention Facility A Flow-through Planter A Dry Well (treatment only) A N/A N/A V N/A N/A Units Notes: A = ft 2 of IMP footprint per ft 2 of tributary impervious area (unitless) V, V 1, V 2 = ft 3 per ft 2 of equivalent tributary impervious area (ft.) STEP 5: OBTAIN SIZING AND RAIN ADJUSTMENT FACTORS FOR EACH IMP For each of the IMPs, obtain the appropriate area sizing factor from Table 4-6. Sizing factors for treatment-only IMPs (in italics) do not require any adjustment for differing rainfall patterns. Both area (A) and volume (V 1, V 2 ) sizing factors for treatment-plus-flow-control IMPs, however, must be adjusted to account for the effects of differing rainfall patterns on pre-project and post-project runoff. 4 th Edition September

61 CHAPTER 4 : LID DESIGN GUIDE Use the equations below to compute the rainfall adjustment: Equation 4-3 For Group A soils, Rain ( 20.2) MAP project site Adjustment = 0.07 Equation 4-4 For Group B soils, Rain Adjustment = ( 20.2) MAP project site 0.11 Equation 4-5 For Group C soils, Rain Adjustment = ( 20.2) MAP project site 0.06 Equation 4-6 For Group D soils, Rain Adjustment = ( 20.2) MAP project site 0.05 where MAP is the mean annual precipitation at the site as shown on the isohyetal map, Contra Costa County Public Works Figure B-166, available on the CCCWP C.3 web pages. STEP 6: CALCULATE MINIMUM AREA AND VOLUME OF EACH IMP The minimum area and storage volumes of each IMP are found by summing up the contributions of each tributary DMA and multiplying by the adjusted sizing factor for the IMP. Equation 4-7 Min. IMP Area DMA DMA IMP Rain or Volume = Square Runoff Sizing Adjustment Footage Factor Factor Factor Bioretention facilities and flow-through planters have two storage volumes. V 1 is the floodable volume above the soil layer. V 2 is the storage volume below the soil layer, calculated by multiplying the volume of gravel by an assumed porosity of 0.4. See Figure 4-6. Note these volumes can be configured in a variety of practical combinations of depth and area to best fit into your landscape design th Edition September 2008

62 CONTRA COSTA CLEAN WATER PROGRAM Cisterns and dry wells have a single storage volume (V). V is calculated using Equation 4-8: Equation 4-8 Min. V DMA DMA IMPVolume Rain = Square Runoff Sizing Adjustment Footage Factor Factor Factor Use the format of Table 4-7 to present the calculations of the required minimum area and volumes of the receiving IMP: TABLE 4-7. Format for presenting calculations of minimum IMP Areas and Volumes DMA Name DMA Area (square feet) Postproject surface type DMA Runoff factor DMA Area runoff factor Soil Type: IMP Name IMP Sizing factor Rain Adjustment Factor Minimum Area or Volume Proposed Area or Volume Total IMP Area V or V1 V2 Orifice Size: STEP 7: DETERMINE IF IMP AREA AND VOLUME ARE ADEQUATE Sizing and configuring IMPs may be an iterative process. After computing the minimum IMP area using Steps 1 6, review the site plan to determine if the reserved IMP area is sufficient. If so, the planned IMPs will meet the Provision C.3 sizing requirements. If not, revise the plan accordingly. Revisions may include: 4 th Edition September

63 CHAPTER 4 : LID DESIGN GUIDE Reducing the overall imperviousness of the project site. Changing the grading and drainage to redirect some runoff toward other IMPs which may have excess capacity. Making tributary landscaped DMAs self-treating or self-retaining (may require changes to grading). Expanding IMP surface area. Once a design with adequate area is achieved, review the IMP configuration to confirm the required minimum volumes are met. If not, revisions to V 1 may include adjusting depth or side slopes and extending the floodable storage area to include adjacent paved or landscaped areas. Revisions to V 2 may include adjusting width or depth, or incorporating buried pipes or arches in the gravel layer. STEP 8: COMPUTER MAXIMUM ORIFICE FLOW RATE This step applies only to treatment-and-flow-control bioretention facilities and flow-through planters built in Group C and Group D soils, and cistern-andbioretention-facilities built in Group B, Group C, and Group D soils. Treatment-only bioretention facilities and flow-through planters in Group C and Group D soils are equipped with underdrains, but there is no restriction on the rate of outflow. For treatment-and-flow-control IMPs, the underdrain has a flow control orifice sized to ensure rates and durations of flows do not exceed pre-project conditions. The cistern-and-bioretention-facility is a hybrid IMP. The flow-control orifice is placed on the outlet from the cistern where it discharges to the bioretention facility. The bioretention facility must have an underdrain in B, C, and D soils, but no flow-control orifice is required on the underdrain. Use the following equations to determine the maximum underdrain flow. Compute the total area draining to an IMP, use the equations to compute the maximum underdrain flow rate, and then apply the orifice equation to estimate the diameter of the flow control orifice th Edition September 2008

64 CONTRA COSTA CLEAN WATER PROGRAM For Bioretention Facilities and Flow-Through Planters: Equation 4-9 Group C soils, Equation 4-10 Group D soils, Equation 4-11 Flow Flow DMA ( cfs) = Square Footage DMA ( cfs) = Square Footage ( 20.2) MAP project site 6 10 ( 20.2) MAP project site Group B soils (cistern-and-bioretention-facility only), Flow DMA ( cfs) = Square Footage ( 20.2) MAP project site To calculate the size of the orifice, use equation Equation 4-12 Orifice Area ( in UnderdrainMaxFlow feet) = c 64.4 H where c is the orifice coefficient, which may be approximated as 0.6. H is the height of the V 2 storage above the orifice. The corresponding orifice diameter may be calculated using Equation Equation 4-13 Orifice Diameter ( in inches) = 12 4 Orifice π Area 4 th Edition September

65 CHAPTER 4 : LID DESIGN GUIDE STEP 9: COMPLETE YOUR SUMMARY REPORT Present your IMP sizing calculations in tabular form. Adapt the following format as appropriate to your project. Coordinate your presentation of DMAs and calculation of minimum IMP sizes with the Stormwater Control Plan exhibit (labeled to show delineation of DMAs and locations of IMPs) and with your Stormwater Control Plan report, which should incorporate a brief description of each DMA and each IMP. Sum the total area of all DMAs and IMPs listed and show it is equal to the total project area. This step may include adjusting the square footage of some DMAs to account for area used for IMPs. Format: Project Name: Project Location: APN or Subdivision Number: Total Project Area (square feet): Mean Annual Precipitation at Project Site: IMPs designed for (treatment only or treatment-and-flow-control): I. Self-treating areas: DMA Name Area (square feet) II. Self-retaining areas: DMA Name Area (square feet) 51 4 th Edition September 2008

66 CONTRA COSTA CLEAN WATER PROGRAM III. Areas draining to self-retaining areas: DMA Name Area (square feet) Postproject surface type Runoff factor Product (Area x runoff factor)[a] Receiving selfretaining DMA Receiving selfretaining DMA Area (square feet) [B] Ratio [A]/[B] IV. Areas draining to IMPs (repeat for each IMP): DMA Name DMA Area (square feet) Postproject surface type DMA Runoff factor DMA Area runoff factor Soil Type: IMP Name IMP Sizing factor Rain Adjustment Factor Minimum Area or Volume Proposed Area or Volume Total IMP Area V or V1 V2 Orifice Size: 4 th Edition September

67 CHAPTER 4 : LID DESIGN GUIDE Specify Preliminary Design Details In your Stormwater Control Plan, describe your features and facilities in sufficient detail to demonstrate the area, volume, and other criteria of each can be met within the constraints of the site. Ensure these details are consistent with preliminary site plans, landscaping plans, and architectural plans submitted with your application for planning and zoning approvals. Following are design sheets for: Self-treating and self-retaining areas Pervious pavements Bioretention facilities Flow-through planter Dry wells and infiltration basins Cistern with bioretention facility These design sheets include recommended configurations and details, and example applications, for these features and facilities. The information in these design sheets must be adapted and applied to the conditions specific to the development project. Local planning, building, and public works officials have final review and approval authority over the project design. Keep in mind that proper and functional design of features facilities is the responsibility of the applicant. Effective operation of facilities throughout the project s lifetime will be the responsibility of the property owner th Edition September 2008

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69 Self-Treating and Self-Retaining Areas CRITERIA Rainfall on self-treating areas infiltrates or during intense storms drains directly off-site or to the storm drain system. Self-retaining areas are designed to retain the first one inch of rainfall without producing any runoff. During intense storms, runoff may drain offsite, to the storm drain system, or to IMPs. LID design seeks to manage runoff from roofs and paving so effects on water quality and hydrology are minimized. Runoff from landscaping, however, does not need to be managed the same way. Runoff from landscaping can be managed by creating self-treating and self-retaining areas. Self-treating areas are natural, landscaped, or turf areas that drain directly off site or to the storm drain system. Examples include upslope undeveloped areas from which runoff is piped or ditched and drained around a development and grassed slopes that drain offsite to a street or storm drain. Self-treating areas may not drain on to adjacent paved areas within the project. Where a landscaped area is upslope from or surrounded by paved areas, a self-retaining area (also called a zerodischarge area) may be created. Self-retaining areas are designed to retain the first one inch of rainfall without producing any runoff. The technique works best on flat, heavily landscaped sites. It may be used on mild slopes if there is a reasonable expectation that the first inch of rainfall would produce no runoff. Areas draining to self retaining areas. Drainage from roofs and paving can be directed to self-retaining areas and allowed to infiltrate into the soil. The maximum ratios are: Best Uses Heavily landscaped sites Advantages No maintenance verification requirement Complements site landscaping Limitations Requires substantial square footage Grading requirements must be coordinated with landscape design Site requirement Treatment only Maximum allowable ratio 2 parts impervious: 1 pervious Stormwater C.3 Guidebook Self-Treating and Self-Retaining Areas September

70 Treatment and flow-control 1 part impervious: 1 pervious The self-retaining area must be bermed or depressed to retain an inch of rainfall including the flow from the tributary impervious area. DETAILS Drainage from self-treating areas must flow to off-site streets or storm drains without flowing on to paved areas within the project. To create self-retaining turf and landscape areas in flat areas or on terraced slopes, berm the area or depress the grade into a concave cross-section so that these areas will retain the first inch of rainfall. Inlets of area drains, if any, should be set 3 inches above the low point to allow ponding. Set overflows and area drain inlets (if any) high enough to ensure ponding (3" deep) over the surface of the self-retaining area. Pavement within a self-treating area cannot exceed 5% of the total area. In self-retaining areas, overflows and area drain inlets should be set high enough to ensure ponding over the entire surface of the self-retaining area. Self-retaining areas should be designed to promote even distribution of ponded runoff over the area. Leave enough reveal (elevation difference) to accommodate buildup of turf or mulch. APPLICATIONS Lawn or landscaped areas adjacent to streets can be considered self-treating areas. Self-retaining areas can be created by depressing lawn and landscape below surrounding sidewalks and plazas. Runoff from walkways or driveways in parks and parklike areas can sheet-flow to self-retaining areas. Roof leaders can be connected to self-retaining areas by piping beneath plazas and walkways. If necessary, a bubble-up can be used. Self-retaining areas can be created by terracing mild slopes. The elevation difference promotes subsurface drainage. Connecting a roof leader to a self-retaining area. The head from the eave height makes it possible to route roof drainage some distance away from the building. Mild slopes can be terraced to create self-retaining areas. 56 September 2008 Self-Treating and Self-Retaining Areas

71 DESIGN CHECKLIST FOR SELF-TREATING AREAS The self-treating area is at least 95% lawn or landscaping (not more than 5% impervious). Re-graded or re-landscaped areas have amended soils, vegetation, and irrigation as may be required to maintain soil stability and permeability. Runoff from the self-treating area does not enter an IMP or another drainage management area, but goes directly offsite or to the storm drain system. DESIGN CHECKLIST FOR SELF-RETAINING AREAS Area is bermed all the way around or graded concave. Slopes do not exceed 4%. Entire area is lawn, landscaping, or pervious pavement (see criteria in Chapter 4). Area has amended soils, vegetation, and irrigation as may be required to maintain soil stability and permeability. Any area drain inlets are at least 3 inches above surrounding grade. DESIGN CHECKLIST FOR AREAS DRAINING TO SELF- RETAINING AREAS Ratio of tributary impervious area to self-retaining area is not greater than 2:1 (1:1 if flow-control requirements apply). Roof leaders collect runoff and route it to the self-retaining area. Paved areas are sloped so drainage is routed to the self-retaining area. Inlets are designed to protect against erosion and distribute runoff across the area. Self-Treating and Self-Retaining Areas September

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73 Pervious Pavements CRITERIA Impervious roadways, driveways, and parking lots account for much of the hydrologic impact of land development. In contrast, pervious pavements allow rainfall to collect in a gravel or sand base course and infiltrate into native soil. Pervious pavements are designed to transmit rainfall through the surface to storage in a base course. For example, a 4-inchdeep base course provides approximately 1.6 inches of storage. Runoff stored in the base course infiltrates to native soils over time. Except in the case of solid pavers, the surface course provides additional storage. Areas with the following pervious pavements may be regarded as self-treating and require no additional treatment or flow control. Pervious concrete Porous asphalt Crushed aggregate (gravel) Open pavers with grass or plantings Open pavers with gravel Artificial turf Solid unit pavers such as bricks, stone blocks, or precast concrete shapes are considered to reduce runoff compared to impervious pavement, when the unit pavers are set in sand or gravel with " gaps between the pavers. Joints must be filled with an open-graded aggregate free of fines. Use the runoff factors in Table 4-2. Areas with pervious pavements can be self-retaining areas receiving runoff from impervious areas if they are bermed or depressed to retain the first one inch of rainfall, including runoff from the tributary impervious area. Best Uses Flat areas Areas with permeable native soils Low-traffic areas Where aesthetic quality can justify higher cost Advantages No maintenance verification requirement Variety of surface treatments can complement landscape design Limitations Initial cost Placement requires specially trained crews Geotechnical concerns, especially in clay soils Concerns about pavement strength and surface integrity Stormwater C.3 Guidebook Pervious Pavements September

74 DETAILS Permeable pavements can be used in clay soils; however, special design considerations, including an increased depth of base course, typically apply and will increase the cost of this option. Geotechnical fabric between the base course and underlying clay soil is recommended. Permeable pavements are best used on grades from flat to approximately 2%. Installations on steeper grades, particularly on clay soils, require cut-off trenches lateral to the slope to intercept, store, and infiltrate drainage from the base course. Pavement strength and durability typically determines the required depth of base course. If underdrains are used, the outlet elevation must be a minimum of 3 inches above the bottom elevation of the base course. Pervious concrete and porous asphalt must be installed by crews with special training and tools. Industry associations maintain lists of qualified contractors. Parking lots with crushed aggregate or unit pavers may require signs or bollards to organize parking. 60 September 2008 Pervious Pavements

75 DESIGN CHECKLIST FOR PERVIOUS PAVEMENTS No erodible areas drain on to pavement. Subgrade is uniform. Compaction is minimal. Reservoir base course is of open-graded crushed stone. Base depth is adequate to retain rainfall and support design loads. If a subdrain is provided, outlet elevation is a minimum of 3 inches above bottom of base course. Subgrade is uniform and slopes are not so steep that subgrade is prone to erosion. Rigid edge is provided to retain granular pavements and unit pavers. Solid unit pavers are set in sand or gravel with minimum " gaps between the pavers. Joints are filled with an open-graded aggregate free of fines. Permeable pavements are installed by industry-certified professionals according to vendor s recommendations. Selection and location of pavements incorporates Americans with Disabilities Act requirements, site aesthetics, and uses. RESOURCES Concrete Promotion Council of Northern California California Asphalt Pavement Association Interlocking Concrete Pavement Institute Start at the Source Design Manual for Water Quality Protection, pp Porous Pavements, by Bruce K. Ferguson ISBN Pervious Pavements September

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77 Bioretention Facilities Bioretention facility configured for treatment-only requirements. Bioretention facilities can rectangular, linear, or nearly any shape. Bioretention detains runoff in a surface reservoir, filters it through plant roots and a biologically active soil mix, and then infiltrates it into the ground. Where native soils are less permeable, an underdrain conveys treated runoff to storm drain or surface drainage. Bioretention facilities can be configured in nearly any shape. When configured as linear swales, they can convey high flows while percolating and treating lower flows. Bioretention facilities can be configured as in-ground or above-ground planter boxes, with the bottom open to allow infiltration to native soils underneath. If infiltration cannot be allowed, use the sizing factors and criteria for the Flow- Through Planter. Best Uses Commercial areas Residential subdivisions Industrial developments Roadways Parking lots Fit in setbacks, medians, and other landscaped areas Advantages Can be any shape Low maintenance Can be landscaped Limitations Require 4%-15% of tributary impervious square footage Require 3-4 feet of head Irrigation typically required CRITERIA For development projects subject only to runoff treatment requirements, the following criteria apply: Parameter Soil mix depth Soil mix minimum percolation rate Soil mix surface area Criterion 18 inches minimum 5 inches per hour minimum sustained (10 inches per hour initial rate recommended) 0.04 times tributary impervious area (or equivalent) Stormwater C.3 Guidebook Bioretention September

78 Parameter Criterion Surface reservoir depth 6 inches minimum; may be sloped to 4 inches where adjoining walkways. Underdrain Required in Group C and D soils. Perforated pipe (PVC SDR 35 or approved equivalent) embedded in gravel ( Class 2 permeable recommended), connected to storm drain or other accepted discharge point. Where flow-control requirements also apply, the bioretention facility must be designed to meet the minimum surface area (A), surface volume (V1), and subsurface volume (V2) using the sizing factors and Equations 4-7 and 4-8. Use check dams for linear bioretention facilities (swales) on a slope. DETAILS Plan. On the surface, a bioretention facility should be one level, shallow basin or a series of basins. As runoff enters each basin, it should flood and fill throughout before runoff overflows to the outlet or to the next downstream basin. This will help prevent movement of surface mulch and soil mix. In a linear swale, check dams should be placed so that the lip of each dam is at least as high as the toe of the next upstream dam. A similar principle applies to bioretention facilities built as terraced roadway shoulders. (a) Parameters for bioretention facilities for treatment and flow-control: A, V1, and V2. 64 September 2008 Bioretention

79 Minimum Surface Volume. For a treatment-andflow-control facility, the sizing factor V 1 is equivalent to the sizing factor A flooded to a 12" depth. Surrounding the facility with a 12" vertical wall minimizes the required surface area as shown in (a). However, alternatives include: Sloping the soil mix surface to be deeper than 12" at the middle, but less deep at the edges, so the average 12" depth is achieved (works best on larger facilities). (b) Sloped side wall Sloping or stepping back the wall as shown in (b) and (c) (requires additional area). Allowing shallow flooding on a portion of adjacent landscape or paving when the facility is at peak capacity as shown in (d) (rare and relatively brief events). Inlets. Paved areas draining to the facility should be graded, and inlets should be placed, so that runoff remains as sheet flow or as dispersed as possible. Curb cuts should be wide (12" is recommended) to avoid clogging with leaves or debris. Allow for a minimum reveal of 4"-6" between the inlet and soil mix elevations to ensure turf or mulch buildup does not block the inlet. In addition, place an apron of stone or concrete, a foot square or larger, inside each inlet to prevent vegetation from growing up and blocking the inlet. (c) Stepped back side wall (d) allowing occasional flooding of adjacent landscaping and pavement. Where runoff is collected in pipes or gutters and conveyed to the facility, protect the landscaping from highvelocity flows with energy-dissipating rocks. In larger installations, provide cobble-lined channels to better distribute flows throughout the facility. Bubble ups can be used to dissipate energy when runoff is piped from roofs and upgradient paved areas. Soil mix. The required soil mix is similar to a loamy sand. It must maintain a minimum percolation rate of 5" per hour throughout the life of the facility, and it Recommended design details for bioretention facility inlets (see text). Bioretention September

80 must be suitable for maintaining plant life with a minimum of fertilizer use. Typically, on-site soils will not be suitable due to clay content. Various local suppliers have identified mixes that meet these criteria. Check with local staff regarding acceptable soil mixes. See Appendix B for further guidance. Storage and drainage layer. Class 2 permeable, Caltrans specification , is recommended. Open-graded crushed rock, washed, may be used, but requires 4"-6" washed pea gravel be substituted at the top of the crushed rock gravel layers. Do not use filter fabric to separate the soil mix from the gravel drainage layer or the gravel drainage layer from the native soil. Minimum subsurface volume. No minimum subsurface volume is required for treatment-only facilities. The gravel layer must be extensive enough and deep enough to ensure the soil mix is well-drained. For treatment-and-flow-control facilities where the native soils are Hydrologic Soil Group C or D, the minimum subsurface volume V 2 in specified in Table 4-6 is equivalent to the minimum area times a 30" deep layer of gravel of 40% porosity. If desired, voids created by buried structures such as pipes or arches may be substituted, as long as the voids are hydraulically interconnected and the minimum subsurface volume calculated by Equation 4-8 is achieved. Buried pipes or arches may be used to achieve the required subsurface volume V2 Underdrains. In locations where native soils beneath the facility are Hydrologic Soil Group A or B, underdrains are optional but municipal reviewers may require them as a preventative against poor drainage. For treatment-only facilities where native soils are Group C or D, a perforated pipe must be bedded in the gravel layer and must terminate at a storm drain or other approved discharge point. For treatment-and-flow-control facilities, the underdrain must be routed through a device designed to limit flows to that specified in Equation 4-9 or Details of combined outletand-underdrain facilities are are shown on pages 70 and 71. Overflow outlets. In treatment-only facilities, overflow outlets must be set high enough to ensure the surface reservoir fills and the entire surface area of soil mix is flooded before the outlet elevation is reached. In swales, this can be achieved with appropriately placed check dams. In treatment-and-flow-control facilities, the outlet elevation must be set to achieve the minimum surface storage volume calculated using Equation 4-8 and the V1 sizing factor. 66 September 2008 Bioretention

81 The outlet should be designed to exclude floating mulch and debris. Vaults, utility boxes and light standards. It is best to locate utilities outside the bioretention facility in adjacent walkways or in a separate area set aside for this purpose. If utility structures are to be placed within the facility, the locations should be anticipated and adjustments made to ensure the minimum bioretention surface area and volumes are achieved. Leaving the final locations to each individual utility can produce a haphazard, unaesthetic appearance and make the bioretention facility more difficult to maintain. Emergency overflow. The site grading plan should anticipate extreme events and potential clogging of the overflow and route emergency overflows safely. Trees. Bioretention areas can accommodate small or large trees within the minimum areas and volumes calculated by Equations 4-7 and 4-8. Tree canopies intercept rain, and extensive tree roots maintain soil permeability and help retain runoff. Normal maintenance of a bioretention facility should not affect tree lifespan. The bioretention facility can be integrated with a tree pit of the required depth and filled with structural soil. If a root barrier is used, it can be located to allow tree roots to spread throughout the bioretention facility while protecting adjacent pavement. Locations and planting elevations should be selected to avoid blocking the facility s inlets and outlets as trees mature. Bioretention facility configured as a tree well. The root barrier is optional. Bioretention facility configured and planted as a lawn/ play area. APPLICATIONS Multi-purpose landscaped areas. Biore tention facilities are easily adapted to serve multiple purposes. The loamy sand soil mix will support turf or a plant palette suitable to the location and a well-drained soil. See Appendix B for additional guidance on soil, plant selection, and irrigation. Bioretention facility configured as a recessed decorative lawn with hardscaped edge. Bioretention September

82 Example landscape treatments: Lawn with sloped transition to adjacent landscaping. Swale in setback area Swale in parking median Lawn with hardscaped edge treatment Decorative garden with formal or informal plantings Traffic island with low-maintenance landscaping Raised planter with seating Bioretention on a terraced slope Residential subdivisions. In the design of many subdivisions, it has proven easiest and most effective to drain roofs and driveways to the streets (in the conventional manner) and then drain the streets to bioretention areas, with one bioretention area for each 1 to 6 lots, depending on subdivision layout and topography. Bioretention areas can be placed on one or more separate, dedicated parcels with joint ownership. Sloped sites. Bioretention facilities must be constructed as a basin or series of basins, with the circumference of each basin level. It may be necessary to add curbs or low retaining walls during final grading if elevations have not been determined with sufficient precision during design. Bioretention facility configured as a parking median. Note use of bollards in place of curbs, eliminating the need for curb cuts. Bioretention facility receiving drainage from individual lots and the street in a residential subdivision. 68 September 2008 Bioretention

83 Design Checklist for Bioretention Volume and depth of surface reservoir meets or exceeds minimum. 18" depth loamy sand soil mix with minimum long-term percolation rate of 5"/hour. Area of soil mix meets or exceeds minimum. Perforated pipe (PVC SDR 35 or approved equivalent) underdrain bedded in Class 2 perm with connection and sufficient head to storm drain or discharge point (except in A or B soils). No filter fabric. Underdrain has a clean-out port consisting of a vertical, rigid, nonperforated PVC pipe, with a minimum diameter of 6 inches and a watertight cap. Location and footprint of facility are shown on site plan, landscaping plan, and grading plan. Bioretention area is designed as a basin (level edges) or a series of basins, and grading plan is consistent with these elevations. If facility is designed as a swale, check dams are set so the lip of each dam is at least as high as the toe of the next upstream dam. Curb inlets are 12" wide, have 4"-6" reveal and an apron or other provision to prevent blockage when vegetation grows in, and energy dissipation as needed. Overflow connected to a downstream storm drain or approved discharge point. Emergency spillage will be safely conveyed overland. Plantings are suitable to the climate, exposure, and a well-drained soil, and occasional inundation during large storm events. Irrigation system with connection to water supply, on a separate zone. Vaults, utility boxes, and light standards are located outside the minimum soil mix surface area. When excavating, avoid smearing of the soils on bottom and side slopes. Minimize compaction of native soils and rip soils if clayey and/or compacted. Protect the area from construction site runoff. For treatment-and-flow-control facilities only Volume of subsurface storage meets or exceeds minimum. In C and D native soils, underdrain is connected via an appropriately sized orifice or other flow-limiting device. Bioretention September

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87 Flow-through Planter Portland 2004 Stormwater Manual Flow-through planters treat and detain runoff without allowing seepage into the underlying soil. They can be used next to buildings and on slopes where stability might be affected by adding soil moisture. Flow-through planters typically receive runoff via downspouts leading from the roofs of adjacent buildings. However, they can also be set in-ground and receive sheet flow from adjacent paved areas. Best Uses Management of roof runoff Next to buildings Dense urban areas Where infiltration is not desired Advantages Can be used next to structures and on slopes Versatile Can be any shape Low maintenance Limitations Can be used for flow-control only on sites with C and D soils Requires underdrain Requires 3-4 feet of head Pollutants are removed as runoff passes through the soil layer and is collected in an underlying layer of gravel or drain rock. A perforated-pipe underdrain must be connected to a storm drain or other discharge point. An overflow outlet conveys flows which exceed the capacity of the planter. CRITERIA Treatment only. For development projects subject only to runoff treatment requirements, the following criteria apply: Stormwater C.3 Guidebook Flow-through Planter September